This series of posts is intended as a basic introduction to the science of evolution for non-specialists. You can see the introduction to this series here. In this post we discuss the current state of hominin paleontology and attempt to locate our lineage within it.
In the last post in this series, we left off the tale of hominin paleontology in the early 1950s, with Piltdown unmasked, and the correct interpretation of Dart’s Australopithecus africanus and Dubois’ Pithecanthropus (Homo) erectus now accepted by the scientific community. Paleontologists now knew where to look (in Africa, as Darwin had suggested) and what to look for (species with human-like bodies and sub-human cranial capacities). In the following decades, many more finds would come to light –revealing the hominins to be a large group with numerous species. Rather than finding a “ladder” of species leading to humans, hominin paleontology would discover a branching tree of our close relatives.
As with all groups of fossil species, determining precise relationships is challenging. As we have pointed out previously in this series, the line between “species” is a fuzzy one, where separation is achieved slowly over time. Widely-separated species are easy to identify as distinct, but as one collects more and more data, the “gaps” start to be filled in, making distinctions more difficult. In a discipline like hominin paleontology this problem is somewhat heightened due to our intense interest in our own lineage – meaning that we have expended great effort to find as many hominin remains as possible. With an abundance of data, the picture becomes harder to resolve in fine detail even as the broad outlines become clearer. This leads to a phenomenon where some paleontologists favor delineating a large number of hominin fossil species, whereas others favor grouping hominin remains into fewer species. Informally, those who favor more species are known as “splitters,” and those who favor fewer species are known as “lumpers.” The fact that such a debate is even possible is testament to the robust, gradient-like data set we have available.
Hominin paleontology: a thumbnail sketch of the current evidence
In broad outline, the current state of hominin paleontology recognizes several large groupings, or grades, with, as we would expect, fuzzy boundaries. A consensus phylogeny representing current thinking is shown below, revealing the hominin family as the bushy tree that it is, and highlighting the abundance of evidence we have accumulated since the time of Dart and Broom:
A hominin phylogeny based on current paleontological – and in some cases, genomic – evidence. Genome sequences are available for humans, Denisovans and Neanderthals. As we have previously discussed, Sahelanthropus tchadensis and Ardipithecus kadabbamay not be a hominins –they might be less closely related to humans than chimpanzees, in which case their positions would be switched with chimpanzees in the phylogeny (indicated with the double-headed arrows). Several hominin species are not shown. See text for details.
Working from the past towards the present, the first grouping we encounter includes species that we have discussed previously – ones that sit close to the human-chimpanzee last common ancestral population. This group, known as “possible and probable hominins” includes the “possible” hominin species Sahelanthropus tchadensis, Orrorin tugenensis, and Ardipithecus kadabba, but only one “probable” hominin, Ardipithecus ramidus. As we noted before, the evidence for bipedality is strongest for Ardipithecus ramidus. While “splitters” recognize these species as distinct, “lumpers” group them together as Ardipithecus ramidus sensu lato, where“sensu lato” indicates “in the broad sense.” In general this grouping exhibits a small braincase volume (under 350 cubic centimeters) and some hints of bipedality.
The second grouping, “archaic hominins” includes the Australopithecines such as Dart’s Australopithecus africanus and an earlier species, Australopithecus afarensis – the most famous example of which is nicknamed “Lucy.” Also included in this general group are the “robust” archaic hominins, the various species assigned to Paranthropus (such as Bloom’s Paranthropus robustus). Generally speaking, in this grouping we see growing evidence of occasional (i.e. facultative) bipedality and a further increase in braincase volume to a range encompassing 500cc. While splitters see numerous additional species in this grouping, lumpers recognize onlyA. afarensis, A. africanus, and one species encompassing all of Paranthropus.
The third grouping, “transitional hominins” contains the oldest-known members of Homo, such as Homo habilis. Underscoring their transitional nature, however, some scientists do not place this species within Homo, but rather describe it as an australopithecine, Australopithecus habilis. Braincase volume in habilis ranges between 500cc and about 700cc. A similar species, Homo rudolfensis, is recognized by splitters but grouped together as H. habilis sensu lato by lumpers. These species (or this species, as the case may be) shows increasing evidence of bipedalism, but not the full commitment to a bipedal lifestyle (i.e. obligate bipedalism) seen in later Homo. These groups also used stone tools of the Oldowan variety, a relatively simple stone tool technology named for the site of its first discovery.
The fourth grouping, “Pre-modern Homo” includes Homo ergaster and Dubois’ Homo erectus, the earliest Homo species that are fully bipedal. While splitters recognize ergaster and erectus as separate species, lumpers place them together as Homo erectus sensu lato. This grade also includes the more familiar Neanderthals as well as the Denisovans, species for which we have full genome sequences. Braincase volume in this grade starts at 700cc for early Homo ergaster/erectus, and extends to the volume seen in modern humans (about 1400cc) (and in fact exceeds average modern human volume in the Neanderthal lineage, where 1600cc skulls are found). Early in this grade, we see advancement to the more complex Acheulean stone tool technology develop.
The last grouping, “modern Homo”, contains only our own species, Homo sapiens, which enters the fossil record about 200,000 years ago. Splitters, however, have argued for an early subspecies, Homo sapiens idaltu, at about 160,000 years ago.
Ancestors, maybe: relatives, certainly
This diversity of remains both complicates the details of our evolution – in that we cannot be absolutely certain which (if any) of these groups are directly ancestral to our own species – and clarifies the overall picture, since these are our close relatives even if not our direct ancestors. Taken together, we can be confident that our lineage passed through these grades in turn – as an ardipithecine (or close relative), to an australopithecine (or close relative), and so on through the transitional hominins and on into pre-modern, and finally modern, Homo. Along the way, our lineage would expand in average braincase volume, acquire the use of ever more sophisticated tools, and, some 200,000 years ago, take our current form in Africa, prior to our expansion across the globe.
In the next post in this series, we’ll take up the story as our species expands out of Africa – and encounters the descendants of related lineages that left Africa before us.