A new analysis of a remarkable hominin find in Ethiopia suggests that the species it represents, Australopithecus anamensis, may be one of the very earliest species in our lineage, and possibly the first hominin we know of that is undoubtedly part of our own genealogy. (“Hominins”, formerly called “hominids”, represent all fossils on our side of the family tree since we branched off from our common ancestor with the chimpanzees). The find has also has led to a revision of the idea that A. anamensis was the ancestor of the later A. afarensis, thought by many to be the ancestor of the genus Homo, and thus of modern Homo sapiens. (We love to know who our ancestors are, as witnessed by the popularity of companies like 23 & Me.)
A. anamensis lived from about 4.2 to 3.9 million years ago (mya) and A. afarensis from 3.9 to 3.0 mya. A. afarensis includes the famous skeleton of Lucy(about 3.2 mya), which is remarkably complete from the neck down but has only fragments of the skull. As the following diagram suggests (and this is typical), A. anamensis is the earliest known hominin to be part of our own lineage and is portrayed as the ancestor of A. afarensis:
That ancestry is presumed (as shown above) to have been a lineal transformation of A. anamensis into A. afarensis: that is, the former species was thought to have evolved over time, and as a unit, into A. afarensis.
This conclusion is what is cast into doubt by the recent find, documented in a new paper in Nature that you can access by clicking on the screenshot below (or see the pdf here). The new finds show that A. anamensis was in fact a contemporary of A. afarensis, so that the two species lived at the same time, at least for a while.
This doesn’t rule out, however, the possibility that A. afarensis evolved from one or more populations of A. anamensis while the latter species continued on, largely unchanged from other populations. (Some, however, would say that such a branching event would automatically change the name of A. anamensis; see below.) Thus both species could still have coexisted while A. afarensis could still be a descendant of A. anemensis.
As you see, the new skull is 3.8 million years old, putting it at about the end of A. anamensis‘s tenure and about the time that A. afarensis appeared.
The new skull is remarkably complete: by far the most intact A. anamensis skull we have, and also has a number of features that tell us that it was a hominin and was not a member of A. afarensis. Since the earliest A. afarensis appears to have lived a bit earlier than this specimen (3.9 mya), it appears that A. afarensis cannot be a lineal descendant of A. anamensis. (But, as I said above, it could be a descendant of some A. anamensis populations.)
Here’s the skull, which has a tiny cranial capacity (365-370 cc compared to about 1200-1300 cc in modern humans), a sagittal crest, and brow ridges.
You can see how small the skull is when compared to the head size of the paper’s first author:
It appears to have been an adult male.
Why is it a hominin? Well, here’s the jargon that they use to show it’s a hominin and more primitive (that is, closer in appearance to the common ancestor) than is A. afarensis:
The specimen is readily identifiable as a hominin by the following morphological features: the canine is reduced in size compared to non-human apes and shows a strong lingual basal tubercle; the mastoids are inflated; the nuchal plane is more horizontal than in non-human apes; and the inion, which is coincident with the opisthocranion, lies near the level of the Frankfort horizontal plane. At the same time, the small cranial capacity, highly prognathic face, extensive pneumatization and other features discussed below indicate that MRD represents a hominin that is more primitive than A. afarensis.
Got that? Neither did I, but the paleoanthropologists do. At any rate, the dates put it earlier than most specimens of A. afarensis, and it has a longer upper canine, a smaller earhole, and a narrower palate. Here’s a comparison of the new specimen (MRD-VP-s/1, which we’ll call MRD) with Sahelanthropus tchadensis (a hominin that may be close to the common ancestor of the human and chimp lineages, and which lived about 7 mya), along with Australopithecus ramidus (a 4.4 mya hominin of unknown placement on the tree), and A. afarensis and the later A. africanus.
Here’s the back of the skull compared to a modern chimp and the two later australopithecines. Note the smaller braincase and more pronounced sagittal crest of MRD:
A phylogenetic analysis of where the MRD skull fits on the hominin phylogeny. As you see, it appears before A. afarensis and sits on part of the phylogeny that gave rise to modern humans.
This species was sexually dimorphic, with males about five feet tall and weighing about 100 pounds (1.5 meters and 45 kilos), and females about 3.5 feet tall and weighing about 62 pounds (1.1 meter and 28 kilos). The skull size and features (tooth wear, etc.) suggest that it was an adult male. Here’s a reconstruction of the face from The Guardian:
The big findings: First, we now know what the cranium of a possible ancestor looked like, and crania are not easily preserved or found in the fossil record of hominins. This helps complete the picture of what A. amanensis looked like. Further, another major and unambiguous conclusion here—assuming the dating is correct—is that A. anamensis coexisted with A. afarensis for at least 100,000 years. A. afarensis is thought to be one of our ancestors, giving rise to the genus Homo as well as the extinct “robust” australopithecines. But this leaves one question:
Was A. anamensis an ancestor of A. afarensis? Just because the two species coexisted does not mean that the one who lived first wasn’t the ancestor of the one that appeared later. Consider that there were various populations of the earlier A. anamensis. Suppose one or more of these evolved into A. afarensis, but some other populations retained the appearance and traits of A. anamensis. Then we’d still have an ancestor/descendant relationship, though cladistic taxonomists would say that at the moment A. afarensis branched off, we’d have to change the name of A. amanensis. (This is part of the practice of cladistic classification, though it makes little sense to laypeople.)
Whether A. afarensis came into being this way or not, what’s clear is that two species of australopithecine lived at the same time, and one of them is thought by paleoanthrophologists, as the first diagram above shows, to have been the ancestor of several species of hominins, including us.