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Linked from this page are documents summarizing the hominid fossil record and hypothesized lines of human evolution from 5 million years ago to the present.
Under the current taxonomy (based on genetic rather than behavioral criteria), the term "hominid" refers to members of the biological human family Hominidae: living humans, all human ancestors, the many extinct members of Australopithecus, and our closest primate relatives, the chimpanzee and gorilla. According to The Tree of Life by Guillaume Lecointre and Hervé Le Guyader (Harvard University Press: 2006), the similarly named and easily confused categories of humans and near human apes, in order of increasing inclusiveness, are:
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 | • Hominini - modern humans and all previous human or australopithicine ancestors
• Homininae - humans and chimpanzees (Panini), our closest living biological kin (so close that some scientists have suggested their genus name should be changed from Pan to Homo). • Hominidae - humans, chimpanzees and gorillas (Gorillinae) • Hominoidae - humans, chimpanzees, gorillas and orangutans (Pongidae) • Hominoidea - humans, chimpanzees, gorillas, orangutans and gibbons (Hylobatoidae). Ardipithecus, the common primate ancestor to paranthropines, australopithicines and humans, went extinct about 4 million years ago. Human evolution is a puzzle made up of thousands of fossil pieces. The Chart of Human Evolution (below) shows the major pieces of that puzzle arranged in a likely solution, which is open to clarification as new fossil or DNA evidence is reviewed in the scientific literature.
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| NOTES
• Each colored bar represents the time interval spanned by recovered fossils associated with that species. Dotted lines indicate the conjectural evolutionary lines of descent. • Under each species name is a list of the national areas where all or most of its fossil remains have been found. • White numbers inside the species bars indicate the approximate count of distinct individuals in each species from whom fossil remains survive. This is considerably smaller than the number of fossil specimens, because a specimen can be a single tooth, bone or bone fragment. • The human fossil record from about 2.5 to 1.0 million years ago is especially sparse — only about 50 individuals are known, many of them represented by only a single tooth or jaw fragment — and the evolutionary connections from australopithecus to homo erectus, including the evolutionary relationships between habilis, ergaster and erectus, are in dire need of clarification. • Time spans for modern humans, Neanderthals and archaic Homo sapiens (H. heidelbergensis) have been extended back beyond accepted fossil limits to accommodate recent genetic evidence that the divergence between the Neanderthal and human lines occurred around 500,000 years ago. • As environmental or climate context, the major Ice Age epochs in recent human experience were [1] the Wisconsin, 11,000-35,000 years ago (the most extreme of recent coolings), and [2] the Illinoian, 130,000-190,000 years ago, with an intermediate ice era around 60,000-70,000 years ago. • Four human species proposed in the literature — H. floresiensis, H. pekinensis, H. georgicus and H. rhodesiensis — have been omitted as conjectural or controversial.
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| Human Variation Across Space and Time | Humans are remarkable for the complexity and pace of their evolutionary history. No other mammal has spread over such a large geographic and ecological range, and evolved so many similar species and radically new forms of behavior, within just two million years.
The origins of this variability are fundamentally genetic — in the human genome — and natural selection produces new species entirely out of the genetic variation within existing species. But the accumulation of early hominid technology and culture gave our biological variability an accelerating push. Before technology could have much impact, evolution was helped along by major cycles of climate change (including changes in sea level), and a human prehistory of migration and the resulting geographic isolation of different hominid groups — as documented in Hominid Fossil Sites and Patterns of Hominid Dispersal. | ||||||||||
| How Many Species Are There? | Radiating into separate geographic or ecological domains, ancestral hominids evolved into regional variants that are often described as different species. Genetic variability within hominid species, and uncertainties in fossil reconstruction or geological dating, make some of the species distinctions controversial.
In all hominids, males are larger than females, and adults are larger than juveniles, and these sex and age differences vary across species. This within species variation complicates the problem of distinguishing one species from another using limited fossil evidence. On these topics, academic discussions are sometimes driven by career, partisan or political considerations. In my view, the fossils and the controversies equally illustrate the physical and cultural variety that is fundamental to human nature. As a further complication, fossils document the coexistence of clearly different hominid species over the last 2 million years — sometimes in adjacent or overlapping geographic regions. The newest finding, published in 2007, indicates that Homo erectus and Homo habilis coexisted in Africa, probably in different ecological niches, for almost 500,000 years. How these different species may have interacted, interbred or contested for resources is unclear.
More recently, the replacement within less than 100,000 years of Homo neanderthalensis by Homo sapiens migrating across Europe may have been passive and opportunistic, as some climate evidence suggests, or it may have been active and aggressive, as modern human behavior (Europeans and American Indians, for example) makes all too plausible. | ||||||||||
| What Is the Human "Family Tree"? | Evolutionary biologists use a cladogram, the treelike diagram of evolutionary branches or clades, to organize species into lines of evolutionary descent across time. A summary picture for human evolution currently lacks key pieces of evidence and for that reason several connections in the diagram above have been omitted.
Biologists use three types of evidence to deduce evolutionary connections: genetics, morphology (behavior), and geologic dating. Different kinds of genetic analysis support the conclusion that global — as opposed to indigenous African — populations diverged about 200,000 years ago (and provides tracking evidence for subsequent waves of human migration out of Africa); that modern humans and extinct neanderthals diverged about 500,000 years ago; and that humans and chimpanzees branched from a common ancestor about 7 million years ago. The rest of the puzzle must be deduced from morphology (physical form, as deduced from the bones) and geologic dating. "Absolute" fossil dating can be quite precise for fossils buried within intact rocks, but for fossils found exposed on the surface, or buried within alluvial or eroded deposits, dates can be grossly conjectural. And morphology becomes a subtle interpretation when the available fossils are crushed and incomplete, or collated from separate geographic locations. The species distinctions and time spans shown in the diagram appear to be reliably documented. The crucial cladogrammic questions currently are these: (1) Is Homo ergaster a distinct species, or an African variant of early Homo erectus? (2) Does Homo ergaster or Homo habilis have priority as the earlier hominid in a linear sequence, or (more likely) were they separate and coexisting hominid lines by 2 million years ago? (3) Is Homo erectus descended from Homo ergaster or is it either (a) descended from Homo habilis or (b) a coexisting branch descended from the same ancestor as Homo habilis? (4) Is archaic Homo sapiens (Homo antecessor or Homo heidelbergensis) descended directly from Homo ergaster and coexisting with Homo erectus, or is it a branching out of Homo erectus?
These questions can only be answered with more complete fossils from the period 2.5 to 1.0 million years ago, which have so far eluded discovery. | ||||||||||
| What Is the Fossil Evidence? | Hominid fossil remains are precious. Complete skeletons are extraordinarily rare before recent times. Teeth and lower jaws, and the facial and upper cranial bones of the skull, are the most common fossils to survive from any period. Skulls are almost never found intact but must be reconstructed from fragments. Thigh bones are next most often retrieved, while remains of the feet, hands, pelvis or spine are extremely rare.
Specific behavioral conclusions require specific parts of the skeleton. For example, adaptation for a crouching or upright posture can be inferred from the connection of the spine to the skull, but bipedalism (habitual walking on two legs) requires evidence from bones involved in the thigh, knee, or foot joints. An opposable thumb requires evidence from wrist or hand bones. Skulls are used as evidence for the evolution of The Hominid Brain. Endocasts (models of the inside of a skull) offer good evidence for the size and shape of the brain that was in the skull, and brain anatomy is sometimes (tenuously) used to infer the cognitive capabilities of the different species; cognitive abilities can also be inferred from the skills required to manufacture Hominid Tools. |
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Last revised 08.17.2007 • © 2007 Bruce MacEvoy | |||||||||||
