The Earliest Human Ancestors: New Finds, New Interpretations
Becky Rogers Ackermann, Ph.D.
Department of Archaeology
University of Cape Town
In the last decade, there has been an explosion of fossil finds of our earliest human ancestors - commonly called "the australopithecines." Most of the finds have been of "gracile", small-bodied specimens, which - unlike the "robust" ones - are generally considered direct ancestors of populations of early
Homo, and by extension direct ancestors of modern humans. Until 1994, all of the known gracile australopithecines were placed in two species:
Australopithecus africanus from South Africa, (see picture to the right)
and Australopithecus afarensis from east Africa (enter here for image). East African
A. afarensis basically stood alone as the candidate for human ancestry from 4 to 3 million years ago (mya), followed by the million year reign (apx 3 to 2 mya) of South Africa's
A. africanus. But today, there are eight putative species of early gracile hominid - and a number of the most recent and most ancient have characteristics that might force a re-interpretation of our earliest ancestry.
What is a gracile australopithecine?
All hominids, including the australopithecines, are bipedal, meaning that they walk on two legs, and are therefore members of the ancestral lineage leading to modern humans (we are the sole bipedal apes). However, our earliest ancestors probably retained some climbing ability, as evidenced by certain traits like curved finger bones, which may indicate that they are not yet fully committed to terrestrial bipedalism; they were, however, capable of it. They are called "gracile" because they generally have small (chimp-sized) bodies and brains. Their development is ape-like, rather than human-like - which is not surprising since human-like development is a derived (rather than primitive) trait - and have an ape-like degree of sexual dimorphism, suggesting ape-like social structure. Their faces are generally large and prognathic, reducing as you move through time towards a flatter more human-like condition. Similarly, their posterior teeth (molars) are big, generally reducing through time, while the tooth enamel thickness increases through time in the lineage. This indicates a transition from a more frugiverous and foliverous diet (a generalized ape condition) to one that is generally omnivorous (like us).
Recent hominid finds
As I mentioned, A. afarensis and A. africanus reigned the day until quite recently - with
A. afarensis discovered in the early 1970's , and A. africanus, the first australopithecine find ever, discovered in 1924 by Raymond Dart . But in 1994 And 1995, two new early hominids were announced -
Ardipithecus ramidus (4.4 mya; see picture to the right) and Australopithecus anamensis (4.2-3.9
mya; see picture to the right), respectively. Both were from east Africa, and both were older than any definitive gracile bodied hominids to date. As the older of the two species, it
made sense that
A. ramidis had more ape-like features, such as relatively thin tooth enamel and large canines, while
A. anamensis displayed a mixture of primitve, ape-like features and derived, human-like features (such as thick tooth enamel) In fact, the primitiveness of
A. ramidus explains why the finders separated it into a different genus
(Ardipithecus), and gave it a specific name (ramidis) which means "root" - e.g. the root of humankind - in the native Ethiopian language.
These rather copious finds (at least by paleoanthropological standards) were followed by three finds of single individuals which were remarkable in part because they altered the perception of where and when these hominids lived. In 1995 a single mandible was described as a new species -
Australopithecus bahrelghazali (3.5 mya) . Despite some dispute over the validity of naming a new species from a single specimen, this individual remains noteworthy because it extends the range of australopithecines to Chad, which lies 2500km west of the Great Rift Valley. The second find comes from Sterkfontein, South Africa, was announced in 1995, and nick-named "Little Foot" because the first bones found were from a foot . Although this yet-to-be-named hominid has still not been fully described (it must be painstakingly removed from a cement-like conglomerate called breccia), faunal associations place it at 3.3 mya - a previously unsampled time period in South Africa. Like the
A. bahrelghazali individual, "Little Foot" extends the geographic range of gracile australopithecines, this time southwards. Both suggest that these hominids were significantly more mobile and versatile than previously understood. The
third hominid -
Australopithecus garhi - was recovered between 1996 and 1998 from the Middle Awash, Ethiopia . Again, the remains appear to be that of a gracile australopithecine, and at approximately 2.5 mya are roughly contemporaneous with South African
A. africanus. What was surprising (garhi means "surprise" in the Afar language) about this find is the existence of lithic technology at 2.5 mya, the possible association of an australopithecine with the lithic technology (tools being largely considered the realm of
Homo), and evidence of food processing in the form of cut and hammerstone marks on long bones.
The most recent early hominid discoveries - all announced this year - are perhaps the most shocking of all, since each of them threatens our previous understanding of who the australopithecines were and how they are related to us. In February 2001, researchers from France announced the discovery of
Orrorin tugenensis, a.k.a. the "Original man" from Tugen Hills, Kenya .
Enter here for image of O.tugenensis. These cranial and post-cranial fragments show direct evidence for bipedalism at 6 million years ago! This is somewhere very close to the molecularly-derived date for human-chimp divergence. Additionally, the discoverers claim that
O. tugenensis has smallish teeth with thick enamel, implying that these are actually primitive traits, retained in
Ardipithecus has thin tooth enamel, they relegate the entire genus to a side branch, possibly of chimpanzee ancestry.
The second discovery came hot on the heels of the first, announced in March 2001.
Kenyanthropus platyops (3.5mya) (enter here for image)
is a flat-faced hominid from Kenya, and is roughly contemporaneous with
A. afarensis. Because of this flat face, other derived facial features, and smaller molars, this genus appears to be closely linked to
Homo, possibly the early ancestor (smaller-brained) of
H. rudolfensis. In the paper describing the specimen, the researchers suggest three major changes to the early hominid tree: changing
Homo rudolfensis into
Kenyanthropus rudolfensis, relegating all big-toothed australopithecines to a side-branch not leading directly to
Homo, and subsequently transferring
A. afarensis to Praeanthropus afarensis. All of these would involve a major re-work of the previous hominid tree.
The third discovery of 2001 does not name a new species - only a new subspecies, for now - but does extend the known existence of
Ardipithecus ramidus significantly back in time to 5.5 mya. Ardipithecus ramidus
kadabba, announced July 2001 is from the Middle Awash, Ethiopia . (Enter
here for image). In the paper describing this find, the author claims that Orrorin was not well evaluated, and that
A. ramidus kadabba is truly the earliest human ancestor, representing the true human lineage. But there is a blanket of uncertainty over all three of these 2001 finds, as we no longer know what an early hominid should look like. Should it have thin enamel or thick? What is derived, and what primitive? Who do we believe?
Where do we go from here?
What we have learned in the last eight years is that our theories about early human ancestry are anything but set in stone. Any of these new fossil finds are poised to alter our understanding of early human ancestry. They have already extended the breadth of our knowledge both in geographic space and evolutionary time. Will they offer more? Only time will tell. But what is becoming increasingly clear is that we are dealing not with a family tree, but with a bush. Or as Darwin proposed - I think more appropriately - a coral, where the base of the coral is long dead, and only the tips live on.
References and Further Reading
1.Johanson DC, White TD, and Coppens Y (1978) A new species of the genus Australopithecus (Primates: Hominidae) from the pliocene of Eastern Africa: Kirtlandia, the Cleveland Museum of Natural History, pp. 1-14.
2. Dart RA (1925) Australopithecus africanus: the man-ape of South Africa. Nature 115:195-199.
3. White TD, Suwa G, and Asfaw B (1995) Ardipithecus ramidus. Nature 375:88.
4. Leakey MG, et al. (1995) New four-million-year-old hominid species from Kanapoi and Allia Bay, Kenya. Nature 376:565-571.
5. Brunet M, et al. (1995) The first australopithecine 2500 kilometres west of the Rift Valley (Chad). Nature 378:273-275.
6. Clarke, RJ and Tobias PV (1995). Sterkfontein Member 2 foot bones of the oldest South African hominid. Science 269;521-524.
7. Asfaw, B. et al. (1999) Australopithecus garhi: a new species of early hominid from Ethiopia. Science 284; 625-629.
8. Pickford, M. & Senut, B. (2001) The geological and faunal context of Late Miocene hominid remains from Lukeino, Kenya. C.R. Acad. Sci. Ser. IIa 332; 145-152.
9. Leakey MG, et al. (2001) New hominin genus from eastern Africa shows diverse middle Pliocene lineages. Nature 410; 433-440.
10. Haile-Selassie, Y. (2001) Late Miocene hominids from the Middle Awash, Ethiopia. Nature 412;178-181.
For More Information:
Dr Becky Rogers Ackermann, firstname.lastname@example.org