The Evolutionary History of Biological Diversity
The Origin and Evolution of of Vertebrates
After completing this unit, students will be able to:
Vertebrates are members of the subphylum, Vertebrata, chordates with backbones and spinal columns. About 58,000 species of vertebrates have been described. Vertebrata is the largest subphylum of chordates, and contains many familiar groups of large land and marine animals. Vertebrates are the animals from the groups of jawless fishes, bony fishes, sharks, stingrays, amphibians, reptiles, mammals, and birds. Extant vertebrates range in size from the carp species, Paedocypris, at as little as 7.9 mm (0.3 of an inch) to the blue whale at up to 33 m (110 feet). Vertebrates make up about 5 percent of all described animal species; the rest are invertebrates, which lack backbones.
The vertebrates traditionally include the hagfish, which do not have proper vertebrae, though their closest living relatives, the lampreys, do have vertebrae. For this reason, the vertebrate subphylum is sometimes referred to as "Craniata," as all members do possess a cranium.
The fossil record shows aspects of the meandering evolutionary path from early aquatic vertebrates to mammals, with a host of transitional fossils, though there are still large blank areas. The earliest known fossil vertebrates were heavily armored fish discovered in rocks from the Ordovician Period, about 500 to 430 (Ma), or million years ago.
The Devonian Period (395 to 345 Ma) brought in the changes that allowed primitive air-breathing fish to remain on land as long as they wished, thus becoming the first terrestrial vertebrates, the amphibians. Amphibians developed forms of reproduction and locomotion and a metabolism better suited for life exclusively on land, becoming more reptilian. Full-fledged reptiles appeared in the Carboniferous Period (345 to 280 Ma). The reptilian changes and adaptations to diet and geography are chronicled in the fossil record of the varying forms of therapsida. True mammals showed up in the Triassic Period (225 to 190 Ma) around the same time as the dinosaurs, which also sprouted from the reptilian line, while birds first diverged from dinosaurs between 100 and 60 Ma.
Figure 1. A series of transverse sections through an embryo of the dog. (After Bonnet.) Section I is the most anterior. In V the neural plate is spread out nearly flat. The series shows the uprising of the neural folds to form the neural canal. a. Aortæ. c. Intermediate cell mass. ect. Ectoderm. ent. Entoderm. h, h. Rudiments of endothelial heart tubes. In III, IV, and V the scattered cells represented between the entoderm and splanchnic layer of mesoderm are the vasoformative cells which give origin in front, according to Bonnet, to the heart tubes, h; l.p. Lateral plate still undivided in I, II, and III; in IV and V split into somatic (sm) and splanchnic (sp) layers of mesoderm. mes. Mesoderm. p. Pericardium. so. Primitive segment.
"Pristella tetra1" by Henry Gray (1827–1861) , Wikimedia Commons is in the Public Domain
The neural crest is unique to craniates. Neural crest cells make up a transient, multipotent, migratory cell population, unique to vertebrates that give rise to a diverse array of cell types.
The emergence of the neural crest was important in vertebrate evolution because many of its structural derivatives are defining features of the vertebrate clade. Several structures that distinguish the vertebrates from other chordates are formed from the derivatives of neural crest cells. Gans and Northcut, in their "New Head" theory, argued that the presence of neural crest was the basis for vertebrate-specific features. The appearance of these features was also pivotal in vertebrate evolution, because it enabled a predatory lifestyle.
Figure 2. Pacific hagfish
"Pacific hagfish Myxine" by U.S. National Oceanic and Atmospheric Administration, Wikimedia Commons is in the Public Domain
Hagfish are marine craniates of the class Agnatha or Myxini, also known as Hyperotreti. Some researchers regard Myxini as not belonging to the subphylum Vertebrata. That is, they are the only living animals that have a skull but not a vertebral column.
Figure 3. Lampreys
"Diversas lampreas.1 - Aquarium Finisterrae" by Drow_male, Wikimedia Commons is licensed under CC BY-SA 3.0
The lamprey ingests the blood of fish by boring holes through tissues with its mouth and tongue.
Figure 4. Shark fish (chondryichthyes)
"Diversas lampreas.1 - Aquarium Finisterrae" by Knepp Timothy, U.S. Fish and Wildlife Service, Wikimedia Commons is in the Public Domain
The Chondryichthyes (cartilaginous fish) are one of the three extant major clades of jawed vertebrates and comprise two sister groups, the elasmobranchs (sharks, skates and rays), and the holocephalans (chimeras and ratfish). Although relatively species-poor by comparison to their osteichthyan (bony vertebrate) relatives (about 970 species of chondrichthyan compared to about 55,600 species of bony vertebrates), they occupy an important phylogenetic position as the sister group to all other jawed vertebrates and as one of the first lineages to diverge after the two rounds of whole-genome duplications that occurred early in vertebrate evolution.
First appearing on Earth almost 450 million years ago, cartilaginous fish today include both fearsome predators and harmless mollusk eaters. Members of chondryichthyes all lack true bone and have a skeleton made of cartilage. Only their teeth, and sometimes their vertebrae, are calcified; this calcified cartilage has a different structure from that of true bone.
Sharks are found in all seas. They generally do not live in fresh water, with a few exceptions such as the bull shark and the river shark which can swim both in seawater and freshwater. Sharks possess a full cartilaginous skeleton and a highly streamlined body, and are perhaps the most successful member of the chondrichthyes (Figure 61). Evidence for the existence of sharks dates from the Ordovician period, more than 420 million years ago, before land vertebrates existed and before many plants had colonized the continents. Only scales have been recovered from the first sharks and not all paleontologists agree that these are from true sharks. The oldest generally accepted shark scales are from about 420 million years ago, in the Silurian period.
Sharks practice internal fertilization. Unlike most bony fish, sharks are K-selected reproducers, meaning that they produce a small number of well-developed young as opposed to a large number of poorly developed young. Unlike bony fish, sharks do not have gas-filled swim bladders for buoyancy. Instead, sharks rely on a large liver, filled with oil that contains squalene and the fact that cartilage is about half as dense as bone. The liver constitutes up to 30 percent of a shark's body mass. The liver's effectiveness is limited, so sharks employ dynamic lift to maintain depth, sinking when they stop swimming. Sand tiger sharks store air in their stomachs, using it as a form of swim bladder. Most sharks need to constantly swim in order to breathe and cannot sleep very long, if at all, without sinking. Certain shark species, like the nurse shark, however, are capable of pumping water across their gills, allowing them to rest on the ocean bottom.
Batoidea is a superorder of cartilaginous fish commonly known as rays and skates, containing more than 500 described species in 13 families. Batoids are flat-bodied, and, like sharks, are a species of cartilaginous marine fish, meaning they have a boneless skeleton made of a tough, elastic substance. Most batoids have five ventral slot-like body openings called gill slits that lead from the gills, but the Hexatrygonidae have six. Batoid gill slits lie under the pectoral fins on the underside, whereas a shark's are on the sides of the head. Most batoids have a flat, disk-like body, with the exception of the guitarfishes and sawfishes, while most sharks have a streamlined body. Many species of batoid have developed their pectoral fins into broad, flat, wing-like appendages. The anal fin is absent. The eyes and spiracles are on top of the head. Batoids have a ventrally located mouth and can considerably protrude their upper jaw (palatoquadrate cartilage) away from the cranium to capture prey. The jaws have euhyostylic-type suspension, which relies completely on the hyomandibular cartilages for support. Most species live on the sea floor, in a variety of geographical regions, many in coastal waters, and a few live in deep waters to at least 3,000 meters (9,800 feet).
Figure 5. Guiya
"Guiyu BW" by Arthur Weasley, Wikimedia Commons is licensed under CC BY 3.0y
Osteichthyes, also called bony fish, are a taxonomic group of fish that have bony, as opposed to cartilaginous, skeletons. The vast majority of fish are osteichthyes, which is an extremely diverse and abundant group consisting of over 29,000 species. It is the largest class of vertebrates in existence today. Osteichthyes is divided into the ray-finned fish (Actinopterygii) and lobe-finned fish (Sarcopterygii).
All bony fish possess gills. For the majority of them, this is their sole or main means of respiration. Lungfish and other osteichthyan species, are capable of respiration through lungs or vascularized swim bladders. Other species can respire through their skin, intestines, and/or stomach.
The Actinopterygii, or ray-finned fishes constitute a class or sub-class of the bony fishes.The ray-finned fishes are so called because they possess lepidotrichia or "fin rays", their fins being webs of skin supported by bony or horny spines ("rays"), as opposed to the fleshy, lobed fins that characterize the class Sarcopterygii which also possess lepidotrichia. These actinopterygian fin rays attach directly to the proximal or basal skeletal elements, the radials, which represent the link or connection between these fins and the internal skeleton (e.g., pelvic and pectoral girdles). In terms of numbers, actinopterygians are the dominant class of vertebrates, comprising nearly 96% of the 25,000 species of fish. They are ubiquitous throughout fresh water and marine environments from the deep sea to the highest mountain streams. Extant species can range in size from Paedocypris, at 8 millimetres (0.31 in), to the massive Ocean Sunfish, at 2,300 kilograms (5,100 lb), and the long-bodied Oarfish, to at least 11 metres (36 ft).
The superclass tetrapoda, or in semi-anglicized form the tetrapods, comprises the first four-limbed vertebrates and their descendants, including the living and extinct amphibians, reptiles, birds, and mammals.
While most species today are terrestrial, there is little evidence that any of the earliest tetrapods could move about on land, as their limbs could not have held their midsections off the ground and the known trackways do not indicate that they dragged their bellies around. Presumably, the tracks were made by animals bottom-walking in shallow water. Amphibians today generally remain semi-aquatic, living the first stage of their lives as fish-like tadpoles. Several groups of tetrapods, such as the snakes and cetaceans have lost some or all of their limbs. And many tetrapods have returned to partially aquatic or (in the case of the cetaceans) fully aquatic lives.
Figure 6. Fossil of Cricotus, an extinct tetrapod.
"Cricotus skull" by Ghedoghedo, Wikimedia Commons is licensed under CC BY-SA 3.0
The nostrils in most bony fish differ from those of tetrapods. Normally, bony fish have four nasal openings, one behind the other on each side. As the fish swims, water flows into the forward pair, across the olfactory tissue and out through the posterior openings. In contrast, tetrapods have only one pair of nares externally, but also sport a pair of internal nares on the palate, allowing them to draw air through the nose. The evolution of internal nares was hotly debated in the 20th century. Basically, the internal nares could be one set of the external ones (usually presumed to be the posterior pair) that have migrated into the mouth, or the internal pair could be a newly evolved structure. A migrating pair of nostrils would however have to cut through the blood vessels and nerves supplying the premaxilla (forward upper jaw), a situation deemed unlikely to have evolved.
The most notable characteristics that make a tetrapod's skull different from a fish's are the relative frontal and rear portion lengths. The fish had a long rear portion while the front was short; the orbital vacuities were thus located towards the anterior end. In the tetrapod, the front of the skull lengthened, positioning the orbits farther back on the skull. The lacrimal bone was not in contact with the frontal anymore, having been separated from it by the prefrontal bone. Also of importance is that the skull was now free to rotate from side to side, independent of the spine, on the newly forming neck.
Amphibians (class Amphibia, from Amphi- meaning on both sides and -bios meaning life) are a class of vertebrate animals, characterized as non-amniote ectothermic (or cold-blooded) tetrapods. Most amphibians undergo metamorphosis from a juvenile water-breathing form to an adult air-breathing form, but some are paedomorphs that retain the juvenile water-breathing form throughout life. Mudpuppies, for example, retain juvenile gills in adulthood.
The three modern orders of amphibians are
Figure 7. Atelopus certus calling male.
"Atelopus certus calling male" by brian gratwicke, Wikimedia Commons is licensed under CC BY 2.0
Figure 8. Shenandoah Salamander
" Shenandoah Salamander" by brian gratwicke, Wikimedia Commons is licensed under CC BY 2.0
Figure 9. Caecilians with its head in burried in the water.
"Caecilian" by Dawson, Wikimedia Commons is licensed under CC BY-SA 2.5
Many amphibians lay their eggs in water. Amphibians are superficially similar to reptiles, but reptiles, like mammals and birds, are amniotes.
Compared to those of amniotes, amphibians' lungs are primitive, possessing few internal septa, large alveoli, and a consequently slow diffusion rate of oxygen into the blood. Ventilation is accomplished by buccal pumping, though most amphibians are able to exchange gases with the water or air via their skin, cutaneous respiration. To enable cutaneous respiration and a sufficient diffusion rate of oxygen, the surface of amphibians' highly vascularized skin must remain moist. Because oxygen concentration in water increases at both low temperatures and high flow rates, aquatic amphibians like the Titicaca water frog or the hellbender salamander can rely primarily on cutaneous respiration. In air, where oxygen is more concentrated, some small species can rely solely on cutaneous gas exchange; these species include the plethodontid salamander, which has neither lungs nor gills. Many aquatic salamanders and all tadpoles have gills in their larval stage, with some, such as the axolotl, retaining gills as aquatic adults.
Most amphibians require freshwater environments in order to reproduce. A few can inhabit brackish water, and some can survive in seawater, but no true marine amphibians exist. Several hundred frog species in adaptive radiations, however, do not need any water for breeding in the wild. They reproduce via direct development, an ecological and evolutionary adaptation that has eliminated their dependency on free-standing water. Almost all of these frogs live in wet tropical rainforests and pass through the tadpole inside the egg. As a result, when these eggs hatch, they yield miniature versions of the adult frog. Reproductive success of many amphibians is dependent not only on the quantity but on the seasonal timing of rainfall.
Several species have also adapted to arid and semi-arid environments, but most still need water to lay their eggs. Symbiosis with single-celled algae that live in the jelly-like layer of the eggs has evolved several times. The larvae of frogs (i.e., tadpoles or polliwogs) initially breathe through exterior gills; later, a pouch forms and covers the gills and the front legs, and lungs form quite early. Newt larvae, which acquire their adult form at an early age, have large external gills that gradually disappear.
Frogs and toads have a distinct tadpole stage, in which they are grazing algae, ongrowth, or filtering plankton until reaching the size necessary for metamorphosis. This metamorphosis typically lasts 24 hours and consists of several changes:
Dramatic declines in amphibian populations, including population crashes and mass localized extinction, have been noted in the past two decades in locations around the world, and amphibian declines are regarded as a particularly critical threat to global biodiversity. The declines are traced to a number of causes, including habitat destruction and modification, over-exploitation, pollution, introduced species, climate change, endocrine-disrupting pollutants, destruction of the ozone layer (ultraviolet radiation has shown to be especially damaging to the skin, eyes, and eggs of amphibians), and diseases like chytridiomycosis. However, many of the reasons for amphibian declines are still poorly understood and remain topics of ongoing discussion. A global strategy to stem the crisis has been released in the form of the Amphibian Conservation Action Plan.
Reptiles are characterized by breathing air, laying shelled eggs (except for some vipers and constrictor snakes that give live birth), and having skin covered in scales and/or scutes. Reptiles are classically viewed as having a "cold-blooded" metabolism. They are tetrapods (either having four limbs or being descended from four-limbed ancestors). Modern reptiles inhabit every continent, with the exception of Antarctica, and four living orders are currently recognized: Crocodilia (crocodiles, gavials, caimans, and alligators), Sphenodontia (tuataras from New Zealand), Squamata (lizards, snakes, and worm lizards), and Testudines (turtles and tortoises).
Unlike amphibians, reptiles do not have an aquatic larval stage. As a rule, reptiles are oviparous[IC69] (egg-laying), although certain species of squamates are capable of giving live birth. This is achieved by either ovoviviparity (egg retention) or viviparity (birth of offspring without the development of calcified eggs). Many of the viviparous species feed their fetuses through various forms of placenta analogous to those of mammals, with some providing initial care for their hatchlings.
All reptiles exhibit some form of cold-bloodedness. This means that most reptiles have limited physiological means of keeping the body temperature-constant, and often rely on external sources of heat. Due to a less-stable core temperature than endothermic birds and mammals, reptilian biochemistry requires enzymes capable of maintaining efficiency over a greater range of temperatures than warm-blooded animals. The optimum body temperature range varies with species, but is typically below that of warm-blooded animals, in the 24-35°C range for many lizards, while extreme heat adapted species like the American desert iguana, Dipsosaurus dorsalis, can have optimal physiological temperatures in the mammalian range, between 35 and 40°C.
Crocodilia
Figure 10. Crocodile head
"Crocodile head" by Steve Hillebrand, U.S. Fish and Wildlife Service, Wikimedia Commons is in the Public Domain
Crocodilia (or Crocodylia) is an order of large reptiles that appeared about 84 million years ago in the late Cretaceous Period (Campanian stage). They are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria. Members of the crocodilian total group, the clade Crurotarsi, appeared about 220 million years ago in the Triassic Period and exhibited a wide diversity of forms during the Mesozoic Era. A crocodile is any species belonging to the family Crocodylidae (sometimes classified instead as the subfamily Crocodylinae). The term can also be used more loosely to include all extant members of the order Crocodilia: i.e. the true crocodiles, the alligators and caimans (family Alligatoridae) and the gharials (family Gavialidae), as well as the Crocodylomorpha which includes prehistoric crocodile relatives and ancestors.
Sphenodontia
Figure 11. Tuatura
"Tuatara" by Andrew McMillan, Wikimedia Commons is in the Public Domain
The tuatara is a reptile endemic to New Zealand which, though it resembles most lizards, is actually part of a distinct lineage, order Sphenodontia. The two species of tuatara are the only surviving members of its order, which flourished around 200 million years ago. Their most recent common ancestor with any other extant group is with the squamates (lizards and snakes). For this reason, tuatara are of great interest in the study of the evolution of lizards and snakes, and for the reconstruction of the appearance and habits of the earliest diapsids (the group that also includes birds, dinosaurs, and crocodiles).
Squamata
Figure 12. Achalinus spilanus
"Juvenile Japanese Odd-scaled Snake" by Unknown, Wikimedia Commons is in the Public Domain
Squamata, or the scaled reptiles, is the largest recent order of reptiles, including lizards and snakes. Members of the order are distinguished by their skins, which bear horny scales or shields. They also possess movable quadrate bones, making it possible to move the upper jaw relative to the braincase. This is particularly visible in snakes, which are able to open their mouths very wide to accommodate comparatively large prey. They are the most variably-sized order of reptiles, ranging from the 16-millimeters (0.63 inches) Jaragua Sphaero (Sphaerodactylus ariasae) to the 8-meter (26 feet) Green anaconda (Eunectes murinus) and the now-extinct mosasaurs, which reached lengths of 14 meters (46 feet) .
Testudines
Figure 13. Terrapin turtle
"Terrapin turtle" by Andrew McMillan, Wikimedia Commons is in the Public Domain
Turtles are divided into two groups, according to how they evolved a solution to the problem of withdrawing their necks into their shells (something the ancestral Proganochelys could not do): the Cryptodira, which can draw their necks in while contracting it under their spine; and the Pleurodira, which contract their necks to the side. Although many turtles spend large amounts of their lives underwater, all turtles and tortoises breathe air, and must surface at regular intervals to refill their lungs. They can also spend much of their lives on dry land. Some species have large cloacal cavities that are lined with many finger-like projections. These projections, called papillae, have a rich blood supply, and increase the surface area of the cloaca. The turtles can take up dissolved oxygen from the water using these papillae, in much the same way that fish use gills to respire.
Birds (class Aves) are feathered, winged, bipedal, endothermic (warm-blooded), egg-laying, vertebrate animals. There are around 10,000 living species of birds, making them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from the 5 cm (2-inch) Bee Hummingbird to the 2.75 m (9 feet) Ostrich. Modern birds are characterized by feathers, a beak with no teeth, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a lightweight but strong skeleton. All living species of birds have wings; the now-extinct flightless Moa of New Zealand was the only exception.
Figure 14. Five ducks swimming on a pond.
"Ducks Swimming" by Michael Ayers, C3BC is licensed under CC BY 3.0
The skeleton consists of very lightweight bones. They have large air-filled cavities (called pneumatic cavities) that connect with the respiratory system. The skull bones in adults are fused and do not show cranial sutures. The orbits are large and separated by a bony septum. The spine has cervical, thoracic, lumbar, and caudal regions with the number of cervical (neck) vertebrae highly variable and especially flexible, but movement is reduced in the anterior thoracic vertebrae and absent in the later vertebrae. The latter few are fused with the pelvis to form the synsacrum. Parts of the vertebral skeleton and braincase are fused to increase its strength while lightening its weight. Most species of bird only possess one ovary rather than two, and no living birds have teeth in their jaw, further reducing body mass. The ribs are flattened and the sternum is keeled for the attachment of large flight muscles except in the flightless bird orders. The forelimbs are modified into wings.
Birds have one of the most complex respiratory systems of all animal groups, as flight requires enormous energy expenditure. Upon inhalation, 75 percent of fresh air bypasses the lungs and flows directly into a posterior air sac that extends from the lungs and connects with air spaces in the bones, filling them with air. The other 25 percent of the air goes directly into the lungs. When the bird exhales, the used air flows out of the lung and the stored fresh air from the posterior air sac is simultaneously forced into the lungs. Thus, a bird's lungs receive a constant supply of fresh air during both inhalation and exhalation. Sound production is achieved using the syrinx-- a muscular chamber incorporating multiple tympanic membranes-- that diverges from the lower end of the trachea.
The bird's heart has four chambers like a mammalian heart. In birds the main arteries taking blood away from the heart originate from the right aortic arch (or pharyngeal arch), unlike in the mammals where the left aortic arch forms this part of the aorta. The postcava receives blood from the limbs via the renal portal system. Unlike in mammals, the circulating red blood cells in birds retain their nucleus.
The nervous system is large relative to the bird's size. The most developed part of the brain is the one that controls the flight-related functions, while the cerebellum coordinates movement and the cerebrum controls behavior patterns, navigation, mating, and nest building. The avian visual system is usually highly developed. Water birds have special flexible lenses, allowing accommodation for vision in air and water. Birds are tetrachromatic, possessing UV-sensitive cone cells in the eye as well as green, red and blue ones. This allows them to perceive ultraviolet light, which is involved in courtship. Many birds show plumage patterns in ultraviolet that are invisible to the human eye; some birds whose sexes appear similar to the naked eye are distinguished by the presence of ultraviolet reflective patches on their feathers. Ultraviolet light is also used in foraging —kestrels have been shown to search for prey by detecting the UV-reflective urine trail marks left on the ground by rodents. Birds with eyes on the sides of their heads have a wide visual field, while birds with eyes on the front of their heads, such as owls, have binocular vision and can estimate the depth of field.
Breeding usually involves some form of courtship display, typically performed by the male. Most displays are rather simple and involve some type of song. Some displays, however, are quite elaborate. Depending on the species, these may include wing or tail drumming, dancing, aerial flights, or communal lekking. Females are generally the ones that drive partner selection, although in the polyandrous phalaropes, this is reversed: Plainer males choose brightly colored females. Courtship feeding, billing, and allopreening are commonly performed between partners, generally after the birds have paired and mated.
Male and female birds have a cloaca, an opening through which eggs, sperm, and wastes pass. Intercourse is performed by pressing the lips of the cloacae together, which is sometimes known as the "cloacal kiss," during which the male transfers his sperm to the female. A few species of birds (e.g. most waterfowl) have an intromittent organ that is known as a phallus and is analogous to the mammal's penis. The female lays amniotic eggs in which the young gestate. Unlike most vertebrates, female birds typically have only one functional ovary and oviduct, and gonads in both species are small except during breeding season--weight-saving adaptations that are useful for flight. They also have no urinary bladder. All birds lay amniotic eggs with hard shells made mostly of calcium carbonate. As a group, birds, like mammals, are noted for their high level of parental care.
Figure 15. Yellow bird perched on a flower.
"Yellow Bird" by Michael Ayers, C3BC is licensed under CC BY 3.0
Mammals (formally Mammalia) are members of a class of air-breathing vertebrate animals characterized by the possession of hair, three middle ear bones, and mammary glands functional in mothers with young. A mammary gland is an exocrine gland: an enlarged and modified sweat gland, which gives mammals their name. In addition to milk production, mammals engage in a high level of prenatal care of their offspring. Most mammals also possess sweat glands and specialized teeth, and the largest group of mammals (placentals) have a placenta that feeds the offspring during gestation. Mammals are endothermic (warm-blooded) and their skin is much thicker than that of birds and often has a continuous layer of insulating fat beneath the dermis.
Figure 16. Otter
"Otter" by Michael Ayers, C3BC is licensed under CC BY 3.0
Mammals are generally considered to be more intelligent than other animals. The mammalian brain, with its characteristic neocortex, regulates endothermic and circulatory systems, including a four-chambered heart.
Mammals belong to an early amniote group called synapsids. Synapsids include mammals and all extinct amniotes more closely related to mammals than to reptiles. Early synapsids had a sprawling posture and a small brain, like most early tetrapods. The parasagittal gait characteristic of most mammals appeared gradually in some therapsids capable of sprawling; this characteristic may have appeared in the hind limb before the fore limb.
Monotremes are mammals that lay eggs (Prototheria) instead of giving birth to live young like marsupials (Metatheria) and placental mammals (Eutheria). Like other mammals, monotremes are warm-blooded with a high metabolic rate (though not as high as other mammals), have hair on their bodies, produce milk through mammary glands to feed their young, have a single bone in their lower jaw, and have three middle-ear bones. The key anatomical difference between monotremes and other mammals is the fact that their urinary, defecatory, and reproductive systems all open into a single duct: the cloaca. This structure is very similar to the one found in reptiles. Monotremes and marsupials have a single cloaca (though marsupials also have a separate genital tract), while placental mammal females have separate openings for reproduction, urination, and defecation: the vagina, the urethra, and the anus. Platypuses and Echnidnas are well known examples of monotremes.
Figure 17. Feeding Platypus
"Feeding Platypus" by Brisbane City Council, Wikimedia Commons is licensed under CC BY 2.0
Figure 18. A wild shortbeak echidna.
"A wild shortbeak echidna," by Fir0002/Flagstaffotos, Wikimedia Commons is licensed under CC BY-NC 3.0
Marsupials are an infra class of mammals, characterized by giving birth to relatively undeveloped young. In the absence of soft tissues, such as the pouch and reproductive system, fossil marsupials can be distinguished from placentals by the form of their teeth; primitive marsupials possess four pairs of molar teeth in each jaw, whereas placental mammals never have more than three pairs.
Like other mammals, marsupials develop their young in a uterus and develop a placenta. Pregnant marsupial females develop a kind of yolk sac in their wombs, which delivers nutrients to the embryo. Marsupials give birth at a very early stage of development (about 4-5 weeks); after birth, newborn marsupials crawl up the bodies of their mothers and attach themselves to a nipple, which is located on the underside of the mother either inside a pouch called the marsupium or open to the environment. The offspring are eventually able to leave the marsupium for short periods, returning to it for warmth, protection and nourishment.
Figure 19. Eastern Grey Kangaroo with joey.
" Eastern Grey Kangaroo with joey" by Fir0002/Flagstaffotos, Wikimedia Commons is licensed under CC BY-NC 3.0
Eutheria are a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals (such as humans) than to living marsupials (such as kangaroos). They are distinguished from non-eutherians by various features of the feet ankles, jaws and teeth.
Humans (known taxonomically as Homo sapiens, Latin for "wise man" or "knowing man") are the only living species in the Homo genus of bipedal primates in Hominidae, the great ape family. Anatomically modern humans originated in Africa about 200,000 years ago, reaching full behavioral modernity around 50,000 years ago.
Human evolution is characterized by a number of important changes — morphological, developmental, physiological, and behavioral — which have taken place since the split between the last common ancestor of humans and chimpanzees. The first major morphological change was the evolution of a bipedal locomotor adaptation from an arboreal or semi-arboreal one, with all its attendant adaptations.
Humans have a highly developed brain, capable of abstract reasoning, language, introspection, and problem solving. The human brain is typically 1,400 cm³ in modern humans, which is over twice the size of that of a chimpanzee or gorilla. This mental capability, combined with an erect body carriage that frees the hands for manipulating objects, has allowed humans to make far greater use of tools than any other living species on earth. Other higher-level thought processes of humans, such as self-awareness, and rationality, are considered to be defining features of what constitutes a "person." The pattern of human postnatal brain growth differs from that of other apes (heterochrony), and allows for extended periods of social learning and language acquisition in juvenile humans.
Other significant morphological changes included the evolution of a power and precision grip, a reduced masticatory system, a reduction of the canine tooth, a shorter digestive tract, and the descent of the larynx and hyoid bone, making speech possible. An important physiological change in humans was the evolution of hidden oestrus, or concealed ovulation, which may have coincided with the evolution of important behavioral changes, such as pair bonding. Another significant behavioral change was the development of material culture, with human-made objects becoming increasingly common and diversified over time.
Humans and chimpanzees diverged approximately six million years ago, making the chimpanzee the closest extant relative to modern humans, having DNA sequences about 98% identical to each other. Through comparison of the two genomes, the genetic differences that have accumulated since the human and chimpanzee diverged from a common ancestor have been cataloged. These changes constitute approximately 35 million single-nucleotide changes, five million ion insertion/deletion events, and various chromosomal rearrangements. Recent molecular studies have shown that there are differences in the expression of 19 regulatory genes between humans and chimpanzees, which many be largely responsible for the differences between the species. Due to the similar nature of the DNA, chimpanzees have been investigated with regard to medical application for humans. Humans and chimpanzees are similar with regard to many aspects of physiology and disease. There are several definite and likely differences, however, such as susceptibility to Alzheimer disease, epithelial cancers, and the progression of HIV to AIDS. Knowledge of chimpanzee biology remains incomplete, and will continue to provide useful information about the human body.
Figure 20. Image showing the relationship between the great apes.
"The Great Apes" by Merrilydancingape, Wikimedia Commons is licensed under CC BY-SA 3.0
Australopithecus is a genus of hominids that is now extinct. Paleontological and archaeological evidence suggests that the Australopithecus genus evolved in eastern Africa around 4 million years ago, before spreading throughout the continent and eventually becoming extinct 2 million years ago. During this period various forms of australopiths existed, including: Australopithecus anamensis, A. afarensis, A. sediba, and A. africanus. Some debate remains among academics whether certain African hominid species of this time, such as A. robustus and A. boisei, constitute members of the same genus; if so, they would be considered robust australopiths, while others would be considered gracile australopiths. If these species do indeed constitute their own genus, then they may be given their own name, the Paranthropus.
It is widely held that the australopiths played a significant role in human evolution. Eventually, one species evolved into the Homo genus (Africa, circa 2 million years ago)—the same genus contained within species like Homo habilis, H. ergaster, and eventually the modern human species, H. sapiens sapiens. Gracile australopiths, widespread throughout Eastern and Northern Africa around 3.5 million years ago, share several traits with modern apes and humans. The earliest evidence of fundamentally bipedal hominids exists at the site of Laetoli in Tanzania. This site contains hominid footprints remarkably similar to those of modern humans, and have been dated to 3.6 million years. Until recently, the footprints were generally classified as australopith since that was the only form of pre-human documented in that region at that time. However, some scholars have considered reassigning the footprints to a yet unidentified very early species of the genus Homo.
The brains of most species of Australopithecus were roughly a third smaller in size than that of a modern human. Also, most species of Australopithecus were diminutive and gracile, standing between 1.2 to 1.4 m tall (approx. 4 to 4.5 feet). Several variations of Australopithecus experience a considerable degree of sexual dimorphism[IC81], in this case males being larger than females. Modern hominids do not display sexual dimorphism to the same degree—in particular, modern humans display a low degree of sexual dimorphism, with males being only 15% larger than females, on average. An Australopithecus male, on the other hand, might possibly have been up to 50% larger than his female counterpart. Despite continuing debate on the subject, new research suggests that sexual dimorphism may not have been this pronounced.
Figure 21. Skull of Australopithecus, with circle representing the size of the brain.
"Skull of Australopithecus africanus (Mrs Ples)" by User:Editor at Large, Wikimedia Commons is in the Public Domain
The fossil record also suggests that Australopithecus is the common ancestor of the distinct group of hominids now called Paranthropus (the "robust australopiths"), and most likely of the genus Homo (that of modern humans). Although the intelligence of these early hominids was likely as sophisticated as modern apes, the bipedal stature[IC82] is the key evidence distinguishing this group from previous quadruped primates. The morphology of Australopithecus contradicts previous theory among scientists that large brains preceded bipedalism. If A. afarensis is definitively the hominid responsible for the footprints at Laetoli, it strengthens the notion that A. afarensis was small-brained but biped. Fossil evidence such as this has made it clear that bipedalism far predated large brains
The postural feeding hypothesis has been recently supported by Dr. Kevin Hunt, a professor at Indiana University. This hypothesis asserts that chimpanzees were only bipedal when they ate. While on the ground, they would reach up for fruit hanging from small trees. While in trees, bipedalism was utilized by grabbing for an overhead branch. These bipedal movements may have evolved into regular habits because they were so convenient in obtaining food. Also, Hunt hypothesises that these movements coevolved with chimpanzee arm-hanging, as this movement was very effective and efficient in harvesting food. When analyzing fossil anatomy, Australopithecus afarensis has very similar features of the hand and shoulder to the chimpanzee, which indicates hanging arms. Also, the Australopithecus hip and hind limb very clearly indicates bipedalism, but these fossils also indicate very inefficient locomotive movement when compared to humans. For this reason, Hunt argues that bipedalism evolved more as a terrestrial feeding posture than as a walking posture. As Hunt says, "A bipedal postural feeding adaptation may have been a preadaptation for the fully realized locomotor bipedalism apparent in Homo erectus."
Another theory on the origin of bipedalism is the behavioral model presented by C. Owen Lovejoy, known as "male provisioning." Lovejoy theorizes that the evolution of bipedalism was linked to monogamy. In the face of long inter-birth intervals and low reproductive rates typical of the apes, early hominids engaged in pair-bonding that enabled greater parental effort aimed at the rearing of offspring. Lovejoy proposes that male provisioning of food would improve the survivorship of the offspring and increase the pair's reproductive rate. Thus, the male would leave his mate and offspring to search for food and return carrying the food in his arms walking on his legs. This model is supported by the reduction ("feminization") of the male canine teeth in early hominids such as Sahelanthropus tchadensis and Ardipithecus ramidus, which along with low body size dimorphism in Ardipithecus and Australopithecus, suggests a reduction in inter-male antagonistic behavior in early hominids. In addition, this model is supported by a number of modern human traits associated with concealed ovulation (permanently enlarged breasts, lack of sexual swelling) and low sperm competition (moderate sized testes, low sperm mid-piece volume) that argues against recent adaptation to a polygynous reproductive system.
However, this model has generated some controversy, as others have argued that early bipedal hominids were instead polygynous. Among most monogamous primates, males and females are about the same size. That is sexual dimorphism is minimal, and other studies have suggested that Australopithecus afarensis males were nearly twice the weight of females (as well as a great deal taller). However, Lovejoy's model posits that the larger range a provisioning male would have to cover (to avoid competing with the female for resources she could attain herself) would select for increased male body size to limit predation risk. Furthermore, as the species became more bipedal, specialized feet would prevent the infant from conveniently clinging to the mother - hampering the mother's freedom and thus make her and her offspring more dependent on resources collected by others. Modern monogamous primates such as gibbons tend to be also territorial, but fossil evidence indicates that Australopithecus afarensis lived in large groups. However, while both gibbons and hominids have reduced canine sexual dimorphism, female gibbons enlarge ('masculinize') their canines so they can actively share in the defense of their home territory. Instead, the reduction of the male hominid canine is consistent with reduced inter-male aggression in a group living primate.
Homo habilis
Figure 22. Skull of Homo habillis, with circle representing the size of the brain.
"Skull of Homo habilis" by User:Editor at Large, Wikimedia Commons is in the Public Domain
Homo habilis ("handy man") is a species of the genus Homo which lived from approximately 2.3 to 1.4 million years ago at the beginning of the Pleistocene period. The discovery and description of this species is credited to both Mary and Louis Leakey, who found fossils in Tanzania, East Africa, between 1962 and 1964. Homo habilis was the earliest known species of the genus Homo until May 2010, when H. gautengensis was discovered, a species believed to be even older than H. habilis. H. habilis was short and had disproportionately long arms compared to modern humans, with a less protruding face than the australopithecines from which it is thought to have descended. H. habilis had a cranial capacity less than half the size of modern humans. Despite the apelike morphology, H. habilis remains are often accompanied by primitive stone tools.
Homo habilis has often been thought to be the ancestor of the more gracile and sophisticated Homo ergaster, which in turn gave rise to the more human-appearing species, Homo erectus. Debates continue over whether H. habilis is a direct human ancestor, and whether all of the known fossils are properly attributed to the species. However, in 2007, new findings suggest that the two species coexisted and may be separate lineages from a common ancestor, instead of H. erectus descending from H. habilis.
Compared to australopithecines, H. habilis's brain capacity of 363 and 600 cm³ was on average 50 percent larger than australopithecines, but considerably smaller than the 1,350 to 1,450 cm³ range of modern Homo sapiens. These hominins were smaller than humans, on average standing no more than 1.3 m (4 feet 3 inches) tall. The small size and rather primitive attributes have led some experts (Richard Leakey among them) to propose excluding H. habilis from the genus Homo, and renaming it as "Australopithecus habilis."
Homo ergaster
Figure 23. Skull of Homo ergaster, with circle representening the size of the brain.
"Skull of Homo ergaster" by User:Editor at Large, Wikimedia Commons is in the Public Domain
Homo ergaster is an extinct chronospecies of Homo that lived in eastern and southern Africa during the early Pleistocene era, about 2.5-1.7 million years ago. There is still disagreement on the subject of the classification, ancestry, and progeny of H. ergaster, but it is now widely accepted to be the direct ancestor of later hominids such as Homo heidelbergensis, Homo sapiens, and Homo neanderthalensis, rather than Asian Homo erectus. It is one of the earliest members of the genus Homo, possibly descended from, or sharing a common ancestor with, Homo habilis. H. ergaster lived in arid habitats and had long legs adapted for long-distance walking. Other features include short straight fingers, indicating they were not tree climbers, and smaller teeth than Australopiths suggesting different foods were eaten or food was prepared by mashing or heating.
Homo ergaster used more diverse and sophisticated stone tools than its predecessor, Homo habilis. H. ergaster refined the inherited Oldowan technology, developing the first Acheulean bifacial axes; while the use of Acheulean tools began ca. 1.6 million years ago, the line of H. erectus diverged some 200,000 years before the general innovation of Acheulean technology.
Sexual dimorphism in H. ergaster is greatly reduced from its australopithecine ancestors (around 20 percent), but still greater than dimorphism in modern humans. This diminished dimorphism is speculated to be a sign of reduced competition for mates between males, which may also correspond to the more modern social practices of ergaster. Not only was H. ergaster like modern humans in body, but also more similar in organization and sociality than any earlier species.
Homo erectus
Figure 24. Skull of Homo erectus, with circle representening the size of the brain.
"Skull of Homo erectus" by User:Editor at Large, Wikimedia Commons is in the Public Domain
Homo erectus (from the Latin ērigere, "to put up, set upright") is an extinct species of hominid that originated in Africa — and spread as far as China and Java — from the end of the Pliocene epoch to the later Pleistocene, about 1.8 to 1.3 million years ago. There is still disagreement on the subject of the classification, ancestry, and progeny of H. erectus, with two major alternative hypotheses: erectus may be another name for Homo ergaster, and therefore the direct ancestor of later hominids such as Homo heidelbergensis, Homo neanderthalensis, and Homo sapiens; or it may be an Asian species distinct from African ergaster. Fossilized remains 1.8 to 1 million years old have been found in Africa, Europe, Indonesia, Vietnam, and China.
H. erectus had a cranial capacity greater than that of Homo habilis: the earliest remains show a cranial capacity of 850 cm³, while the latest Javan specimens measure up to 1,100 cm³, overlapping that of H. sapiens. The frontal bone is less sloped, with the dental arcade smaller than the australopithecines'; and the face is more orthognatic (less protrusive) than either the australopithecines' or H. habilis's, with large brow ridges and less prominent zygomata (cheekbones). These early hominins stood about 1.79 m (5 feet 10 1/2 inches), and were more robust than modern humans.
The Neanderthal is an extinct member of the Homo genus known from Pleistocene specimens found in Europe and parts of Western and Central Asia. Neanderthals are classified either as a subspecies (or race) of modern humans (Homo sapiens neanderthalensis) or as a separate human species (Homo neanderthalensis). The first proto-Neanderthal traits appeared in Europe as early as 600,000-350,000 years ago. Proto-Neanderthal traits are occasionally grouped with another phenetic 'species,'Homo heidelbergensis, or a migrant form, Homo rhodesiensis.
Neanderthal skulls were first discovered in Engis, in what is now Belgium (1829) by Philippe-Charles Schmerling and in Forbes' Quarry, Gibraltar (1848). Another specimen was discovered in a limestone quarry of the Neander Valley in Erkrath near Düsseldorf, Germany, in August 1856—three years before Charles Darwin's On the Origin of Species was published. The type specimen, dubbed Neanderthal 1, consisted of a skull cap, two femora, three bones from the right arm, two from the left arm, part of the left ilium, fragments of a scapula, and ribs.
Early Neanderthals lived in the last glacial period for a span of about 100,000 years. Because the damaging effects the glacial period had on the Neanderthal sites, not much is known about the early species. Remains are known in most European countries south of the line of glaciation, roughly along the 50th parallel north, including most of Western Europe, the south coast of Great Britain, Central Europe and the Balkans, the Ukraine, western Russia, and outside of Europe in the Zagros Mountains and in the Levant.
Neanderthal cranial capacity is thought to have been as large as that of a Homo sapiens, perhaps larger, indicating their brain size may have been comparable, as well. On average, the height of Neanderthals was comparable to contemporaneous Homo sapiens. Neanderthal males stood about 165-168 cm (65-66 inches), and were heavily built with robust bone structure. They were much stronger than Homo sapiens, having particularly strong arms and hands. Females stood about 152-156 cm (60-61 inches) tall.
Neanderthals are thought to have used tools of the Mousterian class, which were often produced using soft hammer percussion—hammers made of materials like bones, antlers, and wood—rather than hard hammer percussion, which used stone hammers. A result of this is that their bone industry was relatively simple. There is good evidence, however, that they routinely constructed a variety of stone implements. Neanderthal tools most often consisted of sophisticated stone-flakes, task-specific hand axes, and spears. Many of these tools were very sharp. There is also good evidence that they used a lot of wood, but wooden objects rarely survive to the present.
Although much has been made of the Neanderthals' burial of their dead, their burials were less elaborate than those of anatomically modern humans. In some cases, Neanderthal burials include grave goods, such as bison and aurochs bones, tools, and the pigment ochre. Neanderthals also performed many sophisticated tasks normally associated only with modern humans. For example, they controlled fire, constructed complex shelters, and skinned animals. A trap excavated at La Cotte de St. Brelade in Jersey gives testament to their intelligence and success as hunters.
Various theories of Neanderthal admixture in modern human DNA—the result of interbreeding of Neanderthals and anatomically modern humans during the Middle Paleolithic—have been debated throughout the 20th century, using genetic research throughout the 2000s. Early theories suggested that Neanderthals were early ancestors of H. erectus, which then evolved into today's H. sapiens. In 2006, however, it became possible to sequence DNA from Neanderthal remains, allowing molecular evidence to show that this was not the case. The results show that Neanderthals interbred with anatomically modern humans.
A draft sequence publication by the Neanderthal Genome Project in May 2010 indicates that Neanderthals share more genetic lineages with non-African populations than with African populations. According to the study, this scenario is best explained by gene flow from Neanderthals to modern humans after humans emerged from Africa. An estimated 1 to 4 percent of the DNA in Europeans and Asians (i.e., French, Chinese and Papua probands) is nonmodern, and shared with ancient Neanderthal DNA rather than with sub-Saharan Africans (i.e., Yoruba and San probands). Though less parsimonious than gene flow, ancient substructure in Africa could account for the higher levels of Neanderthal lineages detected in Eurasians.
Homo sapiens (Humans) are primates of the family Hominidae, and the only extant species of the genus Homo. They originated in Africa, where they reached anatomical modernity about 200,000 years ago, then began to exhibit full behavioral modernity around 50,000 years ago. The human lineage diverged from the last common ancestor with its closest living relative, the chimpanzee, some five million years ago, evolving into the Australopithecines and eventually the genus Homo. The first Homo species to move out of Africa was Homo erectus, the African variety of which, together with Homo heidelbergensis, is considered the immediate ancestor of modern humans. Homo sapiens proceeded to colonize the continents; arriving in Eurasia 125,000-60,000 years ago; Australia around 40,000 years ago; the Americas around 15,000 years ago; and remote islands such as Hawaii, Easter Island, Madagascar, and New Zealand between the years AD 300 and 1280. As early as 12,000 years ago, humans began to practice sedentary agriculture, domesticating plants and animals which allowed for the growth of civilization.
Humans are characterized by having a large brain relative to body size, with a particularly well developed neocortex, prefrontal cortex and temporal lobes; these facilitate abstract reasoning, language, introspection, problem solving, and culture through social learning. This mental capability, combined with an adaptation to bipedal locomotion that frees the hands for manipulating objects, has allowed humans to utilize tools far better than any other living species on Earth.
Human body types vary substantially. Although body size is determined largely by genes, it is also significantly influenced by environmental factors such as diet and exercise. The average height of an adult human is 1.4 m (4 ft 7 in) to 1.9 m (6 ft 3 in), although this varies significantly from region to region and depending on ethnic origin. The average mass of an adult human is 54-64 kg (120-140 lbs) for females and 76-83 kg (168-183 lbs) for males. Weight can also vary greatly (e.g., obesity). Although humans appear hairless compared to other primates, with notable hair growth occurring chiefly on the top of the head, underarms and pubic area, the average human has more hair follicles on his or her body than the average chimpanzee. The main distinction is that human hairs are shorter, finer, and less heavily pigmented than that of the average chimpanzee, making humans harder to see. Humans, like other primates, have sweat glands, better enabling them to conserve energy in tropical environments.
Like all mammals humans are a diploid eukaryotic species. Each somatic cell has two sets of 23 chromosomes, each set received from one parent; gametes have only one set of chromosomes (a mixture of the two parental sets). Among the 23 chromosomes there are 22 pairs of autosomes and one pair of sex chromosomes. Like other mammals, humans have an XY sex-determination system, so that females have the sex chromosomes XX and males have XY. By comparing mitochondrial DNA that is inheritedonly from the mother, geneticists have concluded that the last female common ancestor whose genetic marker is found in all modern humans, the so-called mitochondrial Eve, must have lived around 200,000 years ago. The forces of natural selection have continued to influence human populations, with evidence that certain regions of the genome displayed directional selection within the past 15,000 years. Most current genetic and archaeological evidence supports a recent single origin of modern humans in East Africa, with first migrations placed at 60,000 years ago. Current genetic studies demonstrate that humans on the African continent are the most genetically diverse. However, compared to the other great apes, human gene sequences are remarkably homogeneous.
Life on Earth is a diverse mix of organisms that are all inter-related and yet distinct. The tree of life below represents the relationships between all living things.
Figure 25. A phylogenetic tree of living things, based on RNA data and proposed by Carl Woese, showing the separation of bacteria, archaea, and eukaryotes. Trees constructed with other genes are generally similar, although they may place some early-branching groups very differently, thanks to long branch attraction. The exact relationships of the three domains are still being debated, as is the position of the root of the tree. It has also been suggested that due to lateral gene transfer, a tree may not be the best representation of the genetic relationships of all organisms. For instance some genetic evidence suggests that eukaryotes evolved from the union of some bacteria and archaea (one becoming an organelle and the other the main cell).
"Phylogenetic Tree of Life" by Eric Gaba, Wikimedia Commons is in the Public Domain
Below is a list of terms identified in this module.
Glossary terms from OpenStax College Concepts of Biology licensed under CC-BY 3.0.
This work by Community College Consortium for Bioscience Curriculum is licensed under a Creative Commons Attribution 3.0 Unported License.
Text from BioBook licensed under CC BY-NC-SA, Boundless Biology Open Textbook licensed under CC BY-SA and OpenStax College licensed under CC BY 3.0.
Modified by Courtney A. Harrington, Ph.D. for c3bc.
Instructional design by Nicole P. Lipscomb, M.S., Helen Dollyhite, M.A., Caroline Smith, M.S., Irene Yee Chief, Ph.D. and Courtney A. Harrington, Ph.D. for c3bc.
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