Synapsids

Synapsid, mammal-like reptiles dominated the late Carboniferous and Permian land masses. The first radiation was of the group known as pelycosaurs (Fig. 11.8). This group moved into a range of dry habitats, and evolved to a large size, and in some cases to a herbivorous diet. The best known pelycosaur is Dimetrodon, with a skeleton characterized by a huge sail supported by extensions to its backbone. This sail probably helped the animal to maintain its preferred body temperature, and was one of a range of innovations that contributed to the increase in size and ecological diversity of the group. Descendents of the pelycosaurs, therapsids, radiated widely in the late Permian and extended the domain of tetrapods into much higher latitudes. These organisms were shorter and more squat than the pelycosaurs, and were probably able to generate some of their body heat internally, that is they were probably warm blooded to some degree. Their descendents, the cynodonts, were common in the Triassic, and include the species Thrinaxodon, which shows evidence of having had whiskers. As whiskers are modified hairs, it is probable that the group was furry, which is only of benefit to warm-blooded organisms.

Although mammal-like reptiles were advanced in many ways, they all shared a sprawling gait with their amphibian ancestors. This would have limited their success when competing with a more mobile organism, as many evolving diap-sids were. Synapsids were outcompeted by the ancestors of dinosaurs during the Triassic, and remained a minor group until after the end-Cretaceous extinction event.

Permian mammal-like reptiles

Therapsids

Permian mammal-like reptiles

Therapsids

Pelycosaurs Ancestral synapsid

Marsupials Placental mammals

Marsupials Placental mammals

Jurassic Marsupials

Mammals

A cynodont similar to Thrinaxodon evolved into mammals during the Jurassic. The acquisition of mammalian characters was a patchy affair, and it is difficult to identify a meaningful point where the ancestral group could be said to have evolved into its descendent. The main changes involved are in mobility and jaw structure.

True mammals were the first synapsids to evolve an erect gait, that is, to articulate their limbs at the shoulder and hips. By doing this, they were able to overcome Carrier's constraint, and to move and breathe at the same time. Mammals were also aided in overcoming this problem by the evolution of a diaphragm to protect the lungs from compression.

The jaw array of true mammals has highly specialized teeth and a large cheek, with the chewing action controlled by a major muscle, the masseter, running between the jaws. The lower jaw is composed of one bone, the dentary, that articulates with an upper squamosal bone. In mammal-like reptiles and in diapsids, two extra bones are involved in jaw articulation, the articular and quadrate bones. In mammals these have migrated into the ear where they allow airborne sound waves to be picked up and transmitted to the brain (Fig. 11.9).

The most familiar feature of modern mammals is the ability to bear live young. However, monotremes such as the platypus

(a) Mammalian jaw

(a) Mammalian jaw

Old Maps Featherstone West YorkshireYoung Platypus Diagram
Fig. 11.9 Simplified diagram indicating the main differences in jaw structure between (a) mammals, and (b) their ancestors. These characteristics were acquired gradually and intermediate species can be identified displaying a "mosaic" of primitive and advanced characters.

lay eggs, and marsupial and placental mammals have very different styles of gestation. Many non-vertebrate groups have also independently evolved the ability to give birth to live young. This feature thus assumes less importance in identification of the clade and, possibly, in explaining its Cenozoic success.

Most Mesozoic mammals are known only from their teeth, which have local stratigraphic importance, or from incomplete skulls. They were all small in size and may well have been nocturnal. The majority were predators or omnivores. They included the three subclasses seen today - the monotremes, marsupials, and placental mammals - along with a variety of extinct groups. Most groups were confined to a restricted geographic region. This restriction was enhanced by the break-up of the supercontinent Pangea during the early evolution of mammals. Thus monotremes, which lay eggs, are found only in Australasia; marsupials, which suckle their young in pouches, are found in South America and Australasia; and placental mammals, which retain their young for longer inside the body, are found in Asia, Europe, and North America.

Sixty-five million years ago, dinosaurs became extinct during the end-Cretaceous mass extinction. Within 10 million years, mammals had radiated into almost all of the familiar modern families, including primates and whales. A great deal of parallel evolution occurred, with similar placental and marsupial mammals evolving independently on different continents. A saber-toothed marsupial is known from South America, for example, very like the famous placental saber-tooths of the last ice age.

The first large land animals to colonize the post-dinosaur world were a bizarre set of organisms. They included flightless birds, much larger than modern ostriches, which were predators as well as grazers. Enormous crocodiles, 10 m in length, also competed for the top predator niches. The mammals that radiated alongside these forms were predominantly small carnivores, scavengers, and herbivores. The larger mammalian carnivores that gradually appeared during the Palaeocene and Eocene belonged to an extinct group of mammals known as the creodontids. For the first 10-15 million years of the Cenozoic, faunal interchange on land was extremely limited, leading to the independent evolution of a wide range of mammals in each continent. These radiations included most of the modern groups of mammals, such as modern carnivores, whales, bats, and hoofed grazers.

A major period of climatic cooling began in the Eocene. This had a dramatic effect on vegetation patterns, which caused stress in many terrestrial ecosystems. Falling sea levels, accompanying the development of ice caps at the poles, produced land bridges connecting previously isolated land masses. The net effect of these changes was to cause widespread extinction amongst mammal families, with the survivors being predominantly the modern forms with which we are familiar.

Primates and humans

The order Primates, to which we belong, also includes lemurs, monkeys, and apes, as well as our direct ancestors. The group can be traced back to the late Cretaceous, but radiated in the well-forested, low latitude environments of the early Cenozoic. The ancestors of apes and humans are thought to have evolved about 25 million years ago, at a time when the planet was drying and cooling as ice began to build up at the poles. This trend was even more marked in Africa because of local climatic effects caused by its northward drift and the closure of the Tethyan Ocean to the north.

Around 6 million years ago, as grassland replaced forests, the ancestors of humans moved out onto the plains, and in doing so acquired an upright stance, freeing their upper limbs for carrying food or tools. The earliest of these upright, grassland-dwelling apes were the australopithecines. Footprints have been found in ash falls dated at around 4 million years old, and the oldest well-preserved skeletons, of Australopithecus afarensis, are dated at 3.2 million years ago. Australopithecines evolved in two different ways, towards heavily built vegetarians with small brains, and towards more lightly built omnivores who used simple tools and communal behavior for survival. The more robust lineage is usually known as Paranthropus. Various species of Paranthropus have been identified in Africa, and it is likely that groups of these large vegetarians coexisted with our direct ancestors. The less robust strand of Australopithecus evolved into the genus Homo (Fig. 11.10). This genus evolved through the species Homo habilis, Homo ergaster, and Homo erectus into Homo sapiens, a species that included Neanderthal man as well as ourselves.

As physical changes occurred in the hominin skeleton, a comparable development in behavior is seen, preserved as tools and in the fossil skeletons themselves. Tool making, as distinct from using found objects as tools, is unique to humans, and developed into a progressively more sophisticated form through hominin evolution. Greater ranges of material were employed to construct the tool kit, and the range of functions that tools served increased as well. Analysis of brain shape and the structure of the throat suggests that earlier species of hominin may have been capable of speech, though it may have been more limited in diversity than that acquired by modern humans. The burial of the dead, accompanied by ceremony, is known from Neanderthal sites, and art in a variety of types is known from the sites occupied by modern humans from about 40,000 years ago.

There is great debate about the exact nature of hominin evolution, and much of it is based on sparse evidence. In particular, the timing of hominin migration out of Africa has caused much dispute, as has the point at which the last common ancestor of all humans lived. Figure 11.11 shows the

Fig. 11.10 The evolutionary relationships of primates.

evolutionary relationships of the hominins and basic information about the most important species of hominin.

There is no great "missing link" between apes and humans, and the fossil record, though poor, confirms that our evolution has been similar in all respects to the evolution of trilob-ites, mollusks, or fish. The cooling of Africa and subsequent spread of ice across the northern hemisphere was not simply a background for human evolution, but its crucible.

Out of Africa

There has been a long period of debate about whether modern humans originated in Africa, or from an interbreeding series of populations of Homo erectus that had long since left Africa and radiated across the globe. There is no doubt that these earlier hominins left Africa, but modern genetic studies suggest that they are unlikely to have given rise to modern humans. The genetic variation shown by our species is very small, and it is greatest in Africa. This identifies the area where the species is likely to have evolved first and so had most time to develop a varied genetic identity. Studies of DNA from mitochondria suggest that all living humans may be descended from a group of perhaps 10,000-50,000 people who left Africa around 50,000-100,000 years ago.

Ma r0

Homo neanderthalensis Thick-set humans with strong limbs. Adapted to cold conditions. Widely found in Europe and Near East. Wide diversity of tools. Used fire and caves. Hunted large animals.

Homo sapiens Anatomically modern humans. Known worldwide. Stone-age communities produced tools and artefacts including painted and carved images. Hunter-gatherers.

Homo sapiens Anatomically modern humans. Known worldwide. Stone-age communities produced tools and artefacts including painted and carved images. Hunter-gatherers.

Homo neanderthalensis

Homo sapiens

Homo erectus (China)

Homo erectus (South-east Asia)

Homo ergaster Similar body proportions to modern humans. Used advanced tools including hand-axes and cleavers. Charred animal bones suggest creative use of fire. Known only from Africa.

Tools Made Homo Habilis

Homo habilis Limbs and body proportions similar to modern humans. May have had speech. Made tools (mainly chert or flint flakes) and used them for hunting and scavenging. Only known from Africa.

Homo erectus Physically similar to modern humans. Probably nomadic. Produced tools that included axe heads and spears. Cave dwelling and made fires. Known from Africa, Asia and Europe.

Paranthropus boiset

Paranthropus boisei Fully adapted to life on the ground. Only known from Africa. Had a good grip and probably used found objects as tools.

Vegetarian diet.

Homo ruddifensis

Homo habilis Limbs and body proportions similar to modern humans. May have had speech. Made tools (mainly chert or flint flakes) and used them for hunting and scavenging. Only known from Africa.

AusStalopithhccs aaricaaus

Australopithecus aethiopicus

Australopithecus afarenscs (including A. anamenscs)

Australopithecus ramcdus

Australopithecus afarensis Many ape-like characteristics. Probably lived on the ground and in trees. Able to walk upright. Only known from Africa. May have used found objects as tools. Vegetarian diet.

Fig. 11.11 Evolutionary relationships of hominins.

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