Chapter 19 Primate and Human Evolution Oldest Hominid Sahelanthropus tchadensis the oldest known hominid nearly 7 million years old was discovered in 2002 in Chad Who are we Who are we Where did we come from What is the human genealogy These are basic questions that probably everyone at some time or another has asked themselves Goes Back Farther Than We Thought Many people enjoy tracing their own family history as far back as they can similarly paleoanthropologists are discovering based on recent fossil finds that the human family tree goes back much farther than we thought Hope of Life In fact a skull found in the African nation of Chad in 2002 and named Sahelanthropus tchadensis but nicknamed Tourma which means hope of life in the local Goran language has pushed back the origins of humans to nearly 7 million years ago Instead of simplifying our ancestry however its discovery has raised more questions than it answered Bushy Model of Human Evolution For instance paleoanthropologists now think that human evolution branched many times rather than evolving in a somewhat straight line leading to modern humans According to this bushy model of human evolution such key traits as upright walking manual dexterity and a large brain evolved more than once and produced many evolutionary dead ends When Humans and Chimpanzees Diverged Presently most paleoanthropologists accept that the human chimpanzee stock separated from gorillas about 8 million years ago and humans separated from chimpanzees about 5 million years ago Thus Sahelanthropus tchadensis is at or near the point in time when humans and chimpanzees diverged Oldest Hominid Besides being the oldest hominid humans and their extinct ancestors Sahelanthropus tchadensis shows a mosaic of primitive and advanced features that has excited and puzzled paleoanthropologists The small brain case and most of the teeth except the canines are chimplike However the nose which is fairly flat and the prominent brow ridges are features only seen until now in the human genus Homo Understanding in Flux So where does this leave us evolutionarily speaking At a very exciting time as we seek to unravel the history of our species Our understanding of our genealogy is presently in flux and each new fossil hominid find sheds more light on our ancestry Human Evolution Apparently human evolution is just like that of other groups Just as with nonhominid predecessors our ancestors followed an uncertain path As new species evolved they filled ecologic niches and either gave rise to descendants better adapted to the changing environment or became extinct New Hypotheses About Our Ancestry In this section we examine the various primate groups in particular the origin and evolution of the hominids the group that includes our ancestors However we must point out that new discoveries of fossil hominids as well as new techniques for scientific analysis are leading to new hypotheses about our ancestry Continuing Discoveries Change Our Ideas As recently as 2000 the earliest fossil evidence of hominids was from 4 4 million year old rocks in eastern Africa Since then as just noted discoveries have pushed that age back to almost 7 million years Exciting Study Even as we speak therefore new discoveries may have changed some of our conclusions based on what we currently know Such is the nature of paleoanthropology and one reason why the study of hominids is so exciting What Are Primates Primates are difficult to characterize as an order because they lack the strong specializations found in most other mammalian orders We can however point to several trends in their evolution that help define primates and are related to their arboreal or tree dwelling ancestry Trends in Primates These include changes in the skeleton and mode of locomotion an increase in brain size a shift toward smaller fewer and less specialized teeth and the evolution of stereoscopic vision and a grasping hand with opposable thumb Not all these trends took place in every primate group nor did they evolve at the same rate in each group Variations In fact some primates have retained certain primitive features whereas others show all or most of these trends Classification of Primates The primate order is divided into two suborders The prosimians or lower primates include the lemurs lorises tarsiers and tree shrews while the anthropoids or higher primates include monkeys apes and humans Classification of Primates Order Primates Suborder Prosimii lower primates Lemurs lorises tarsiers tree shrews Suborder Anthropoidea Higher primates Monkeys apes humans Superfamily Cercopithecoidea Macaque baboon proboscis monkey Superfamily Ceboidea Howler spider and squirrel monkeys Superfamily Hominoidea Apes humans Family Pongidae Chimpanzees orangutans gorillas Family Hylobatidae Gibbons siamangs Family Hominidae Humans Prosimians Prosimians are generally small ranging from species the size of a mouse up to those as large as a house cat They are arboreal have five digits on each hand and foot with either claws or nails and are typically omnivorous They have large forwardly directed eyes specialized for night vision hence most are nocturnal Tarsier Tarsiers are prosimian primates Ring Tailed Lemur Ring Tailed Lemur are also prosimians Prosimians As their name implies pro means before and simian means ape prosimians are the oldest primate lineage and their fossil record extends back to the Paleocene During the Eocene prosimians were abundant diversified and widespread in North America Europe and Asia Eocene Prosimian Notharctus a primitive Eocene prosimian from North America Prosimians Declined in Cooler Climate As the continents moved northward during the Cenozoic and the climate changed from warm tropical to cooler midlatitude conditions the prosimian population decreased in both abundance and diversity Prosimians Are Tropical By the Oligocene hardly any prosimians were left in the northern continents as the once widespread Eocene populations migrated south to the warmer latitudes of Africa Asia and Southeast Asia Presently prosimians are found only in the tropical regions of Asia India Africa and Madagascar Anthropoids Anthropoids evolved from a prosimian lineage sometime during the Late Eocene and by the Oligocene they were well established Anthropoids are divided into three superfamilies New World Monkey New World Monkeys constitute a superfamily belonging to the suborder Anthropoidea anthropoids Old Word Monkey Another