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:

• 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).

The chart (at right) shows the evolutionary chronology inputed to these biological branches. 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.

The tentative connections between species or time of extinction, indicated by a "?", are open to clarification as new DNA and fossil evidence is reviewed in the scientific literature; see comments below the chart.

• 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.

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 variety in the fossils, and the controversies about how the variety should be interpreted, illustrate the physical and cultural divergences that characterize 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. A report published in 2007 demonstrates 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.

Biologists use three types of evidence to deduce evolutionary connections: genetics, morphology, and geologic dating. (Behavior, normally a key part of evolutionary studies, cannot be observed in extinct species.) Different kinds of genetic analyses support the conclusion that humans and chimpanzees branched from a common ancestor about 7 million years ago. DNA recovered from an uncontaminated fossil indicates that modern humans and extinct neanderthals diverged about 500,000 years ago. Recent genetic samples collected from indigenous populations around the world indicate that the ancestors of the world human population diverged from the indigenous African population about 200,000 years ago. These studies also provide remarkably detailed evidence for subsequent waves of human migration, including a final migration out of Africa around 90,000 years ago by the first humans similar to ourselves.

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 cladogram for human evolution shown above currently lacks key pieces of evidence. For that reason several descent connections in the diagram above have been omitted, although the species distinctions and time spans shown in the diagram appear to be adequately documented.

These are some of the most interesting but currently unanswered questions about human descent:

(1) 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 about 2 million years ago?

(2) Is Homo ergaster a distinct species, or an African variant of early Homo erectus?

(3) Are the (mostly African) examples of Homo erectus descended from Homo ergaster; or are they either (a) descended from Homo habilis or (b) descended from the same ancestor as Homo habilis?

(4) Do the (mostly Asian) fossils of Homo erectus represent a single species that lived for 2 million years or a sequence of species flourishing around 1.6 million, 1.2 million and 200,000 years ago?

(5) 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?

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 make fire and manufacture Hominid Tools.