The Line To Humans

Early primates_

The Primates is one of the oldest of the modern placental mammal groups. The name "primate" (from primus, "first") does not refer to this, but to the fact that humans are primates, and so "first" among animals: the namer, Homo sapiens ("wise person"), has the privilege of choosing the name! For most of their history, the primates were a rare and rather obscure group. All primates share a number of features that give them agility in the trees (mobile shoulder joint, grasping hands and feet, sensitive finger pads), a larger than average brain, good binocular vision and enhanced parental care (one baby at a time, long time in the womb, long period of parental care, delayed sexual maturity, long lifespan).

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Figure 17.17 Diverse euarchontoglirans: (a) the Eocene rodent Paramys; (b) the Paleocene primate Plesiadapis; and (c) the Miocene ape Proconsul. (a, based on Wood 1962; b, based on Lewin 1999.)

Early relatives of the primates such as Plesi-adapis (Fig. 17.17b) were squirrel-like animals that may have climbed trees and fed on fruit, seeds and leaves. Various basal primates radiated in the Paleocene, Eocene and Oligocene, and gave rise to the modern lemurs, lorises and tarsiers.

True monkeys arose in the Eocene and they diverged into two groups, the New World monkeys of South America, and the Old World monkeys of Africa, Asia and Europe. The New World monkeys, such as marmosets and spider monkeys, have flat noses and prehensile tails that may be used as extra limbs in swinging through the trees. The Old World monkeys, such as macaques and baboons, have narrower projecting noses and non-prehensile tails, or no tails at all.

The apes arose from the Old World monkeys before the end of the Oligocene and the group radiated in Africa in the Miocene. Even early forms, such as Proconsul (Fig. 17.17c), have no tail, and a relatively large braincase, indicating high intelligence. These apes ran about on the ground and along low branches on all fours, and fed on fruit. The apes spread out from Africa into the Middle East, Asia and southern Europe by the mid-Miocene and gave rise to some of the modern ape groups at that time. Fossil and molecular evidence on phylogeny (see pp. 133-4) suggests that the gibbons of Southeast Asia are the most primitive living apes, having branched off 2520 Ma, followed by the orangutan 20-15 Ma (Fig. 17.18). The focus of ape (and human) evolution remained in Africa.

Figure 17.18 Phylogeny of the primates, showing some of the main fossil and living groups (a), and the detail of one view of human evolution (b). A., Australopithecus; H., Homo; M, Mesozoic; Q, Quaternary.

Figure 17.18 Phylogeny of the primates, showing some of the main fossil and living groups (a), and the detail of one view of human evolution (b). A., Australopithecus; H., Homo; M, Mesozoic; Q, Quaternary.

Gorillas, chimpanzees and humans appear to be very closely related: they share many anatomic characters and more than 94% of their DNA is identical. Gorillas seem to have diverged first, about 10 Ma, and the ancestors of humans and chimps separated about 8-6 Ma.

Human evolution

Humans are set apart from other primates by their large brain and their bipedalism, walking upright. We might like to think the large brain and human intelligence evolved first, but all the evidence shows that the move from four to two legs came first, and this may have been because of a climatic accident in Africa in the late Miocene. Much of Africa had been covered with lush forests in which the ancestral apes flourished, but climates then became arid and the East African rift valley began to open up, separating the forests in the west from the arid grasslands in the east. Tree-living apes (chimps and gorillas) retreated west and the remaining apes (our ancestors) remained in the eastern grasslands. They had to stand upright to look for enemies, to permit them to run long distances in search of food and to free their arms for carrying food. Humans evolved from bipedal ape-like ancestors that had no special high brainpower.

Until recently, the oldest human remains were about 4 Ma, which seemed to match the molecular estimate of 5 Ma for the split between humans and chimps. Then, in 2001 and 2002, there was a flurry of excitement when two French teams announced human fossils that were 6 Ma. There was immediate dispute about whether these fossils truly were human, and which was oldest, and the debate rumbles on (Box 17.7).

A number of incomplete human fossils were reported during the 1990s from Africa from rocks dated between 6 and 4 Ma. It may be that Sahelanthropus and Orrorin were already bipedal by 6 Ma, but the oldest clear evidence for bipedalism was the find of some human tracks in volcanic ash from Tanzania, dated at 3.75 Ma. The oldest substantial skeletons, of Praeanthropus afarensis, come from rocks dated at about 3.2 Ma and also show clear anatomic evidence for advanced biped-alism, but still an ape-sized brain. The famous skeleton of a female P. afarensis from Ethio pia, called Lucy by its discoverer Don Johan-son in the 1970s (Fig. 17.20a), has a rather modern humanoid pelvis and hindlimb (Fig. 17.20b). The pelvis is short and horizontal, rather than long and vertical as in apes, the thighbone slopes in towards the knee, and the toes can no longer be used for grasping. Lucy's brain, however, is small, only 415 cm3 for a height of 1-1.2 m - not much different from a chimpanzee.

The human genus Australopithecus continued to evolve in Africa from about 3 to 1.4 Ma, giving rise to further small species, and some large robust ones (Fig. 17.21a, b). The larger australopithecines reached heights of 1.75 m, but their brain capacities did not exceed 550 cm3, a rather ape-like measure. The leap forward to modern human brain sizes only came with the origin of a new human genus, Homo. The first species, H. habilis (Fig. 17.21c), lived in Africa from 2.4 to 1.5 Ma, and had a brain capacity of 630700 cm3 in a body only 1.3 m tall. H. habilis may have used tools. It is a remarkable fact that, for over 1 myr, three or four different human species lived side by side in Africa.

So far, the focus of human evolution had been entirely in Africa, but a new species, H. erectus, which arose 1.9 Ma in Africa, spread to China, Java and central Europe. H. erectus had a brain size of 830-1100 cm3 (Fig. 17.21d) in a body up to 1.6 m tall, and there is clear evidence that this early human species had semipermanent settlements, a basic tribal structure, knew about the use of fire for cooking, and made tools and weapons from stone and bone.

Modern peoples_

Truly modern humans, H. sapiens, may have arisen as much as 400,000 years ago, and certainly by 150,000 years ago, in Africa, having evolved from H. erectus. It seems that all modern humans arose from a single African ancestor, and that the H. erectus stocks in Asia and Europe died out. H. sapiens spread to the Middle East and Europe by 90,000 years ago. The European story is particularly well known, and it includes a phase, from 90,000 to 30,000 years ago, when Neandertal man occupied much of Europe from Russia to Spain and from Turkey to southern England. Neandertals had large brains (on average,

Box 17.7 Vive la différence!

The world of paleoanthropology was rocked in 2001 when a team based in Paris, Senut et al. (2001), announced a new hominid, Orrorin tugenensis from Kenya, dated as 6 Ma. The remains were rather scrappy, as hominid remains often are: teeth, jaw fragments and limb bones. Brigitte Senut and her colleagues argued that the teeth were rather ape-like, and that the arm bones suggested Orrorin could brachiate - that is, swing through the trees arm-over-arm, like an ape. However, the femur (thigh bone) showed that Orrorin stood upright, and so this was a true early human. Other paleo-anthropologists questioned these claims: they accept the great age of the fossils, but some have doubted whether the remains all belong to the same species, and others question whether Orrorin really could have walked upright.

Soon after, another French team, this time from Poitiers, announced a second ancient hominid, from Chad in North Africa: Sahelanthropus (Brunet et al. 2002). Sahelanthropus is based on a fairly complete skull (Fig. 17.19), some fragmentary lower jaws and teeth. The sediments in Chad are hard to date, but are about 7-6 Ma, perhaps the same as Orrorin, or maybe older. The Sahelanthropus skull indicates a brain volume of 320-380 cm3, similar to a modern chimpanzee, but the teeth are more human-like, with small canines. The position of the foramen magnum is disputed: Michel Brunet and his colleagues claim the foramen magnum, the great opening in the skull through which the spinal cord passes, is located beneath the skull, so implying that Sahelanthropus stood upright. Critics have said the specimen is too shattered to be sure, and that the foramen magnum might lie more towards the back of the skull, so suggesting that the spinal cord exited horizontally and that Sahelanthropus stood on all fours. These critics have cheekily suggested Sahelanthropus ("Sahel man") should be renamed Sahelpithecus ("Sahel ape")!

Read more in the original papers by Senut et al. (2001) and Brunet et al. (2002), and at many web sites, linked from http://www.blackwellpublishing.com/paleobiology/.

Figure 17.19 Our oldest ancestor? The spectacular skull of Sahelanthropus from the upper Miocene of Chad, over 6 Ma. (Courtesy of Michel Brunet.)

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Figure 17.20 The origin of bipedalism in humans: (a) the Pliocene hominid Praeanthropus afarensis, known as "Lucy"; and (b) comparison of the hindlimb of an ape (left), Lucy (middle) and a modern human (right). (Based on Lewin 1999.)

Figure 17.20 The origin of bipedalism in humans: (a) the Pliocene hominid Praeanthropus afarensis, known as "Lucy"; and (b) comparison of the hindlimb of an ape (left), Lucy (middle) and a modern human (right). (Based on Lewin 1999.)

1400 cm3), heavy brow ridges (Fig. 17.21e) and stocky powerful bodies. They were a race of H. sapiens adapted to living in the continuous icy cold of the last ice ages, and had an advanced culture that included communal hunting, the preparation and wearing of sewn animal-skin clothes, and religious beliefs. Some paleoanthropologists see the

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Figure 17.21 Skulls of fossil humans in front and side views: (a) Australopithecus africanus; (b) A. boisei; (c) Homo habilis; (d) H. erectus; (e) H. sapiens, Neandertal type; and (f) modern H. sapiens. (Based on Lewin 1999.)

Figure 17.21 Skulls of fossil humans in front and side views: (a) Australopithecus africanus; (b) A. boisei; (c) Homo habilis; (d) H. erectus; (e) H. sapiens, Neandertal type; and (f) modern H. sapiens. (Based on Lewin 1999.)

Box 17.8 The quest for the Hobbit

An extraordinary human fossil was unearthed in 2003 on the island of Flores, Indonesia, in Southeast Asia. The bones came from a woman who was fully grown, and yet barely 1 m tall, and she was named as the exemplar of a new human species, Homo floresiensis. This is an astonishing claim, that a distinct human species existed in Indonesia at the same time as H. sapiens was striding across northern Asia towards the Bering Strait, and Middle Stone Age peoples in Europe were painting wonderful hunting scenes in the caves of France.

The Flores remains consist of a more or less complete skull and skeleton, as well as the remains of seven other individuals. The skull (Fig. 17.22) shows this hominid had a brain size of only 380 cm3. It was named as a clearcut new species, H. floresiensis, by its discoverers, Peter Brown of the University of New England, Australia, and colleagues (2004). They argued that the remains show evidence for a unique population of little people - soon dubbed the hobbits by the press - that, remarkably, hunted pygmy elephants and giant lizards on the island. The discoverers argued that this unique species had lived on the island for some time, unaware of its larger human relatives elsewhere, and that Flores man had become small as a result of island dwarfing. It is a relatively common observation that mammals isolated on islands may become smaller (or sometimes larger) because there are usually fewer species than on the neighboring mainland, and each species finds a new place in the ecosystem. The elephants on Flores doubtless became small too so they could survive in a smaller area.

As soon as the paper appeared, many paleoanthropologists were immediately critical. Surely this could not be a distinct species, merely a local variant of H. sapiens, either a pygmy race, or perhaps a microcephalic individual (Jacob et al. 2006), that is an individual with an unusually small head and brain? There have been accusations that researchers on both sides of the dispute have been less than willing to share the specimens, that others have borrowed specimens and then refused to hand them back, that specimens have been damaged, and other multifarious examples of skullduggery. The majority view still seems to be that H. floresiensis is truly a distinct species, but the inaccessibility of the location and legal problems may make further collecting difficult.

Read more about the hobbit in Morwood and van Oosterzee (2007) and Culotta (2007), as well as web presentations available through http://blackwellpublishing.com/paleobiology/.

Figure 17.22 Skulls of Flores man, Homo floresiensis (left), and of a typical modern human, H. sapiens, to show the great difference in size. (Courtesy of Paul Morwood.)

Neandertals as distinct enough to be given their own species, H. neanderthalensis.

The Neandertals disappeared as the ice withdrew to the north, and more modern humans advanced across Europe from the Middle East. This new wave of colonization coincided with the spread of H. sapiens (Fig. 17.21f) over the rest of the world, crossing Asia to Australasia before 40,000 years ago. There is much debate about the dating of these migrations, and how the various human populations are related. New discoveries from Indonesia have stirred up real controversy over whether there was a unique small-sized human species, H. floresiensis, living there only 18,000 years ago (Box 17.8).

Equally controversial is the question of when modern humans reached the Americas. All agree that native Americans walked across from Siberia to Alaska, and colonized southwards over some hundreds or thousands of years. It is commonly said that humans reached North America 11,500 years ago, and yet apparently older remains are reported from time to time. These fully modern humans, found worldwide, with brain sizes averaging 1360 cm3 (see Fig. 17.21f), brought more refined tools than those of the Neandertals, art in the form of cave paintings and carvings, and religion. The nomadic way of life began to give way to settlements and agriculture about 10,000 years ago.

1 Why were dinosaurs so huge? Establish the typical size range of dinosaurs in comparison with modern mammals, and read about ideas past and present about why dinosaurs were an order of magnitude larger than mammals.

2 Were dinosaurs warm-blooded? Read around the topic, back to the debates in the 1970s and 1980s, and through to the present day. List the different lines of evidence used to suggest endothermy and evaluate the arguments for and against.

3 Did mammals radiate explosively after the Cretaceous-Tertiary event? Investigate the "classic" story of a massive diversification/adaptive radiation of mammals 65 Ma, and consider why molecular phy-logenies and dates seem to indicate a much earlier diversification.

4 What happened to mammalian faunas 11,000 years ago, at the end of the ice ages? Read about the climate change and overkill hypotheses for Pleistocene extinctions, and decide which side of the debate has the best evidence.

5 Track the discoveries of new human fossils over the past 20 years, and focus on the question of dating human origins. What were the oldest human fossils in 1980, 1990 and 2000, and what is the view today?

Further reading

Benton, M.J. 2005. Vertebrate Paleontology, 3rd edn. Blackwell, Oxford.

Brunet, M., Guy, F., Pilbeam, D. et al. 2002. A new hominid from the upper Miocene of Chad, central Africa. Nature 418, 145-51.

Carroll, R.L. 1987. Vertebrate Paleontology and Evolution. Freeman, San Francisco.

Chiappe, L.M. 2007. Glorified Dinosaurs: The origin and early evolution of birds. Wiley-Liss, New York.

Cracraft, J. & Donoghue, M.J. 2004. Assembling the Tree of Life. Oxford University Press, New York.

Culotta, E. 2007. The fellowship of the Hobbit. Science 317, 740-2.

Delson, E., Tattersall, I., Van Couvering, J.A. & Brooks, A.S. 2002. Encyclopedia of Human Evolution and Prehistory, 2nd edn. Garland, New York.

Erickson, G.M., Curry-Rogers, K. & Yerby, S. 2001. Dinosaur growth patterns and rapid avian growth rates. Nature 412, 429-33.

Farlow, J.O. & Brett-Surman, M.K. (eds) 1997. The Complete Dinosaur. Indiana University Press, Bloomington.

Fastovsky, D.E. & Weishampel, D.B. 2005. The Evolution and Extinction of the Dinosaurs, 2nd edn. Cambridge University Press, Cambridge.

Gould, S.J. (ed.) 2001. The Book of Life. Norton, New York.

Kemp, T.S. 2005. The Origin and Evolution of Mammals. Oxford University Press, Oxford.

Lewin, R. 2004. Human Evolution, 5th edn. Blackwell, Oxford.

Lewin, R. & Foley, R. 2003. Principles of Human Evolution, 2nd edn. Blackwell, Oxford.

Luo, Z.-X. 2007. Transformation and diversification in early mammal evolution. Nature 450, 1011-19.

Morwood, M. & van Oosterzee, P. 2007. A New Human. Smithsonian Books, Washington, DC.

Reisz, R.R., Scott, D., Sues, H.-D. et al. 2005. Embryos of an Early Jurassic prosauropod dinosaur and their evolutionary significance. Science 309, 761-4.

Rose, K.D. & Archibald, J.D. (eds) 2005. Placental Mammals: Origin, Timing and Relationships of the

Review questions

Major Extant Clades. Johns Hopkins University Press, Baltimore.

Sander, P.M. & Klein, N. 2005. Developmental plasticity in the life history of a prosauropod dinosaur. Science 310, 1800-2.

Sander, P.M., Mateus, O., Laven, T. & Knötschke, N. 2006. Bone histology indicates insular dwarfism in a new Late Jurassic sauropod dinosaur. Nature 441, 739-41.

Savage, R.J.G. & Long, M.R. 1986. Mammal Evolution. British Museum (Natural History), London.

Senut, B., Pickford, M., Gommery, D. et al. 2001. First hominid from the Miocene (Lukeino Formation, Kenya). Comptes Rendus de l'Academie de Sciences 332, 137-44.

Stringer, C.B. & Andrews, P. 2005. The Complete World of Human Evolution. Thames & Hudson, London.

Tabuce, R., Asher, R.J. & Lehmann, T. 2008. Afrothe-rian mammals: a review of current data. Mammalia 72, 2-14.

Weishampel, D.B., Dodson, P. & Osmolska, H. (eds) 2004. The Dinosauria, 2nd edn. University of California Press, Berkeley.

Wood, B. & Richmond, B.G. 2007. Human Evolution: A guide to fossil evidence. Westview Press, Boulder, CO.

Zhou, Z.-H., Barrett, P.M. & Hilton, J. 2003. An exceptionally preserved Lower Cretaceous ecosystem. Nature 421, 807-14.

Asher, R.J. 2007. A web-database of mammalian morphology and a reanalysis of placental phylogeny. BMC Evolutionary Biology 7, 108. doi 10.1186/1471-2148-7-108.

Brown, P., Sutikna, T., Morwood, M.J. et al. 2004. A new small-bodied hominin from the Late Pleistocene of Flores, Indonesia. Nature 431, 1055-61.

Brunet, M., Guy, F., Pilbeam, D. et al. 2002. A new hominid from the upper Miocene of Chad, central Africa. Nature 418, 145-51.

Carpenter, K. 1982. Skeletal and dermal armor reconstruction of Euoplocephalus tutus (Ornithischia: Ankylosauridae) from the Late Cretaceous Oldman Formation of Alberta. Canadian Journal of Earth Sciences 121, 689-97.

Estes, R. 1983. Sauria terrestria, Amphisbaenia. Handbuch der Paläoherpetologie 10A, 1-249. Gustav Fischer, Stuttgart.

Fraser, N.C. & Walkden, G.M. 1984. The postcranial skeleton of the Upper Triassic sphenodontid

Planocephalosaurus robinsonae. Palaeontology 27, 575-95.

Gregory, W.K. 1951/1957. Evolution Emerging, Vols 1, 2. Macmillan, New York.

Hu, Y., Meng, J., Wang, Y. & Li, C. 2005. Large Meso-zoic mammals fed on young dinosaurs. Nature 433, 149-52.

Jacob, T., Indriati, E., Soejono, R.P. et al. 2006. Pygmoid Australomelanesian Homo sapiens skeletal remains from Liang Bua, Flores: population affinities and pathological abnormalities. Proceedings of the National Academy of Sciences, USA 103, 13421-6.

Jepsen, G.L. 1970. Biology of Bats. Vol. 1. Bat Origins and Evolution. Academic, New York.

Lewin, R. 1999. Human Evolution, 4th edn. Blackwell, Oxford.

Newman, B.H. 1970. Stance and gait in the flesh-eating dinosaur Tyrannosaurus. Biological Journal of the Linnean Society 2, 119-23.

Ostrom, J.H. 1969. Osteology of Deinonychus antir-rhopus, an unusual theropod from the Lower Cretaceous of Montana. Bulletin of the Peabody Museum of Natural History 30, 1-165.

Rinderknecht, A. & Blanco, R.E. 2008. The largest fossil rodent. Proceedings of the Royal Society B 275, 923-8.

Senut, B., Pickford, M., Gommery, D. et al. 2001. First hominid from the Miocene (Lukeino Formation, Kenya). Comptes Rendus de l'Académie de Sciences 332, 137-44.

Springer, M.S., Cleven, G.C., Madsen O. et al. 1997. Endemic African mammals shake the phylogenetic tree. Nature 388, 61-4.

Springer, M.S., Murphy, W.J., EiZirik, E. & O'Brien, S. J. 2003. Placental mammal diversification and the Cretaceous-Tertiary boundary. Proceedings of the National Academy of Sciences, USA 100, 1056-61.

Thewissen, J.G.M., Cooper, L.N., Clementz, M.T., Bajpai, S. & Tiwari, B.N. 2007. Whales originated from aquatic artiodactyls in the Eocene epoch of India. Nature 450, 1190-4.

Thewissen, J.G.M., Hussain, S.Y. & Arif, M. 1994. Fossil evidence for the origin of aquatic locomotion in archaeocete whales. Science 263, 210-12.

Wood, A.E. 1962. The early Tertiary rodents of the family Paramyidae. Transactions of the American Philosophical Society 52, 1-261.

Zhou, Z.-H., Barrett, P.M. & Hilton, J. 2003. An exceptionally preserved Lower Cretaceous ecosystem. Nature 421, 807-14.

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