Both
Sahelanthropus and
Orrorin existed during the estimated duration of the ancestral chimpanzee–human speciation events, within the range of eight to four million years ago (Mya). Very few fossil specimens have been found that can be considered directly ancestral to genus
Pan. News of the first fossil chimpanzee, found in Kenya, was published in 2005. However, it is dated to very recent times—between 545 and 284 thousand years ago. The divergence of a "proto-human" or "pre-human" lineage separate from
Pan appears to have been a process of complex
speciation-
hybridization rather than a clean split, taking place over the period of anywhere between 13 Mya (close to the age of the tribe Hominini itself) and some 4 Mya. Different
chromosomes appear to have split at different times, with broad-scale
hybridization activity occurring between the two emerging lineages as late as the period 6.3 to 5.4 Mya, according to Patterson et al. (2006), This research group noted that one hypothetical late hybridization period was based in particular on the similarity of
X chromosomes in the proto-humans and stem chimpanzees, suggesting that the final divergence was even as recent as 4 Mya. Wakeley (2008) rejected these hypotheses; he suggested alternative explanations, including
selection pressure on the X chromosome in the ancestral populations prior to the
chimpanzee–human last common ancestor (CHLCA). Most
DNA studies find that humans and
Pan are 99% identical, but one study found only 94% commonality, with some of the difference occurring in
non-coding DNA. It is most likely that the australopithecines, dating from 4.4 to 3 Mya, evolved into the earliest members of genus
Homo. In the year 2000, the discovery of
Orrorin tugenensis, dated as early as 6.2 Mya, briefly challenged critical elements of that hypothesis, as it suggested that
Homo did not in fact derive from australopithecine ancestors. All the listed fossil genera are evaluated for two traits that could identify them as hominins: • probability of being ancestral to
Homo, and • whether they are more closely related to
Homo than to any other living primate. Some, including
Paranthropus,
Ardipithecus, and
Australopithecus, are broadly thought to be ancestral and closely related to
Homo; others, especially earlier genera, including
Sahelanthropus (and perhaps
Orrorin), are supported by one community of scientists but doubted by another. ==List of known hominin species==