Evolutionary tree-phylogenies can be constructed only with those species that are recognized. Currently, the group of recognized hominid species is substantially incomplete. Some argue existing fossils may represent species other than those already formally named. The probable incompleteness of the fossil record should be borne in mind whenever a diagram is offered as hypothesis of the hominid phylogeny.
Robert Martin recently calculated about 6000 primate species have ever existed The 183 living primate represent 3% of the total. The family trees constructed today probably have half their branches missing. It is hard to avoid the conclusion that under current taxonomic practice there is a distinct tendency to underestimate the abundance of species in the primate, and notably the hominid fossil record. The taxonomic practice Ian Tattersall refers has to do with the interpretation of anatomical variability.
During the first half of the century, it was common to apply a new species name to virtually each new fossil unearthed. Each variant in anatomical structure was taken as indicating a separate species, an approach known as splitting. The result was a plethora of names in the hominid record.
In 1965, Elwyn Simmons and David Pilbeam rationalized this paleontological mess and reduced the number of genera and species to a mere handful. This approach, in which anatomical differences are interpreted as variation within species rather than variation between species, is known as lumping. Lumping became the guiding ethic of anthropology. This notion explained all anatomical differences among hominid fossils at any point in time as within species variation. In other words, only 1 hominid species existed at any one time, making the hominid family tree a progression of species through time, each following on from the one before. The true hominid family tree, the one that actually happened in evolutionary history almost certainly is more bushy than the ones currently drawn. Many are accepting that hominid phylogeny is more complex than portrayed. This was emphasized by the rethinking by the discovery of the Black skull in 1985. A robust australopithecine that did not immediately fit into the prevailing phylogenetic picture.
The hominid lineage probably evolved 10 and 5 million years ago. The first point to make is that the earliest known species is Australopithecus afarensis have been dated between 3.7 and 2.8 million years ago. All other hominid species are dated later than 3 million years ago. It is possible that afarensis was the first member of the hominid lineage and that no other species evolved until after 3 million years ago. Given the evolutionary pattern of mammalian groups, this seems unlikely. The typical evolutionary pattern is that once a new lineage is established there follows a quite rapid radiation of species. The group or clade is bushy from the beginning, not just half way through. It remains to be seen whether A. afarensis is the tip of an existing hominid bush, other branches having become extinct. The fossil gap between 3.7 and 5 to 8 million years ago will be difficult to fill because in much of their anatomy-especially their cranial and dentition these early hominid will be difficult to distinguish from the earliest ancestors of the modern African apes. The great majority of anthropologist view A. afarensis as the stock from which all later hominids derived. Current debate centers on the exact course of these deviations. One clear trend through time in the hominid lineage was the increase in the size of cheek teeth and reduction in sizes of the front teeth, particular the canines. There are 2 min proposed phylogeny-both have certain difficulties.
Although both procedures are known to occur in evolution, they represent special cases. Although proposal 2 with A. africanus as the last common ancestor was popular for some years, just recently it has been overtaken by events, specifically the discovery in 1985 of the Black Skull. Part of the structure of proposal 2 was a continual progression of dental robusticity through time in the australopithecines, from A. africanus to A. robustus to A. boisei. However the discovery of the Black Skull ruled out the notion of a simple progression because although it was very close to the beginning of the lineage in time it was already extremely robust. Moreover, the new fossil had striking similarities in the rear of the cranium with A. afarensis. There is therefore a reasonable argument to be made that the new fossil which some people call A. aethiopicus evolved directly from A. afarensis and was then ancestral to the other robust australopithecine species. Inevitably, this pushes A. africanus to one side. A. africanus would remain as the most likely ancestor of Homo habilis but would no longer be ancestral to the robust australopithecines. A phylogeny of this sort is the one currently more favored. Although this discussion has focus on the origin of Homo habilis as a major branch of the hominid phylogeny, several scholars believe there to have been at least 2 contemporary Homo species between 2.5 and 1.6 million tears ago.