Hominid Species Time Line

The most important characteristic of the genus Homo is larger brain size compared to other hominids, plus a tendency (with a few exceptions) to evolve larger brains over time. The second most important characteristic, no doubt related to the first, is the association of cultural artifacts—tools—with members of this genus. In discussions of these key cladistic features of the genus in the mid-twentieth century, scientists established a threshold of 750 cc as the theoretical boundary between apes and humans’ brains. It was also confidently assumed at that time that members of our genus were the sole makers of tools.

These clear markers have been effaced by subsequent discoveries. In the first place, species in the genus Homo do tend to have larger brains, and their brains increase in size over time. But the earliest species proved not to be as “brainy” as expected, so the line between humans and australopithecines is less clear than originally imagined. What prompted this increase in brain size in our genus is still an open question. There are many theories about what drove the changes, but no universally accepted answer.

Regarding the second criterion, the earliest known stone tools date to 2.6 million years ago or even earlier, but have not been definitively associated with a particular species. It seems probable, in fact, that these tools were made by australopithecines, A. Garhi, and not by a Homo species. [Recent observations of tool-making by chimps should cure us of the disposition to claim that species of our genus are the sole makers of tools.]

In the absence of such clear taxonomic distinctions, controversy has flourished, and scientists have fallen back on subtler genus markers, such as teeth and jaw characteristics, the configuration of the brain, and evidence of development of a capability for speech.

Unfortunately, one of the most frustrating gaps in the fossil record involves the scarcity of hominid remains from the critical 3 million to 2.3 million year-old strata—the era, presumably, when the first members of the genus Homo emerged. We have evidence of a few australopithecine species at that time, but little trace of our Homo line.

A few teeth, bone fragments, and a palette from this period, 2.3 million years ago (see image below), look like an early species in the genus Homo, perhaps H. habilis, but the evidence is inconclusive. The palette on the left is ape-like in shape with pronounced canine teeth. Definitely not a human. The specimen on the right, however, from the Hadar region of Ethiopia, has the shape and teeth characteristics of a possible human ancestor—short face and square palette. In fact, the teeth and palette look very human.

In addition to these uncertainties about the earliest members of our genus, there is disagreement about whether early specimens constitute separate species or represent examples of variations within a species. This argument is particularly crucial in regard to the earliest species/specimens of the genus Homo. Experts in the field are divided between “clumpers” and “dividers”—that is, those who regard the variations in specimens as representing differences within a single species (clumpers) and those who regard the variations as indicative of several distinct species living and competing in the same region at the same time (dividers). Ian Tattersall and Donald Johanson can be taken to represent these two contending views. The differences among specimens that Tattersall interprets as requiring separate species designation, Johanson often sees as representing variations within a single species. DNA evidence would provide clinching evidence in this argument, but is not likely to be available. [See Ian Tattersall, “Once We Were Not Alone,” Scientific American, (January, 2000), pp. 56-62.]

Even if early human fossils from this time horizon, 2.3 to 3 million years ago, are discovered in the future, some degree of uncertainty is likely to remain. In the absence of DNA evidence, lines of descent must be inferred from anatomical similarities. Physical remains can often be dated with some degree of precision, and it is possible to establish a fairly reliable chronology, including relative or even absolute time relationships among specimens. Study of physical remains also establishes anatomical similarities between specimens and groups of specimens and therefore identifies likely lines of descent. But similarities in structure establish degrees of probability about relatedness and lines of descent without the certainty that we would desire. Without DNA evidence we are obliged to wait for a fuller fossil record.

Below are sample specimens of individuals classified as human species, members of the genus Homo, and they are arranged left to right in the chronological order in which they lived.


H. rudolfensis	Homo habilis	Heidelbergensis	Neandertalensis

As the genus Homo emerged, there was apparently a proliferation of species at the outset, and tangled, bush-like set of family relationships—much as in the larger hominid family. There are as yet few clear or obvious “straight lines” of descent, but branching and parallel developments and some dead ends that leave us uncertain about aspects of the human lineage.

Classification of the currently known remains is also highly contested. We have reason to expect from existing human populations that considerable variation existed among members of earlier human (Homo) species, and we must keep in mind that species are not absolute entities, especially viewed over time. With only a small number of partial specimens from which to draw inferences, the challenges facing scientists in this field are truly daunting. Imagine that anthropologists of the future had fossil remains of only two individuals of the current human population, and one was Shaq O’Neill and the other Danny Devito.

According to the “dividers,” there are at least three current candidates for the position of earliest human species, H. rudolfensis, H. habilis, and H. ergaster. The specimens discovered thus far and grouped in these categories (over protests!) exhibit distinct physical differences—facts that would seem to support the “dividers.” But the likelihood that two, and possibly three, similar species co-existed and competed in the same ecological niche in the same region (East Africa’s Rift Valley) at the same time provokes skepticism on the part of the “clumpers.” Add Homo georgicus in the Caucasus region at nearly 2 million years of age and Homo erectus (or an undefined predecessor) in China even earlier, and we have an almost incredibly complicated situation to sort out, without a clear view of what led to this situation. Did I mention that one of the oldest finds — @ 2.25 million years ago — is in China?

We will have to wait for new evidence to understand the full implications of the situation around 2 million years ago.