Dr Alison Campbell
Department of Biological Sciences
School of Science and Technology Phone +64 7 838 4732
The University of Waikato E-mail
Private bag 3105 www.waikato.ac.nz
Hamilton, New Zealand.
Human evolution: the roots and twigs of the tree.
There is no straight-line evolutionary path to modern humans. Current thinking sees our branching tree beginning 5 – 7 mya with the last common ancestor of apes and humans & ending with Homo sapiens.
Recent discoveries in our fossil past have focussed attention on these two ends of our fossil tree: the ancient and the recent.
At the deep end: Sahelanthropus tchadensis (6 – 7 mya), Orrorin tugenensis (6mya) & Ardepithecus ramidus (4.4 mya): in some ways these fossils raise more questions than they answer about this period in our history.
Sahelanthropus: a single skull, unearthed in 2002: there are no postcranial remains. The skull shows a mosaic of ape-like and hominid features, including a surprisingly flattened face. March 2005 computer-aided reconstruction suggests it was bipedal. Ape or human?
Orrorin (2001): largely postcranial remains, with some teeth and jaw fragments; there is no cranium. Again, it is the focus of discussion about its affinities (e.g. Aiello & Collard 2001). Was it bipedal? Recent analyses of the neck of the femur suggest Orrorin was bipedal: bone density was not equal at all points but showed the unequal distribution characteristic of other bipeds, and this is used to justify a relationship with the hominins. This is supported by the thick enamel and relatively small size of the teeth.
The first remains of Ardepithecus ramidus were discovered in 1997, but further finds (the most recent reported in January this year) have brought the number of known individuals to >50. The post-cranial skeleton and position of the foramen magnum indicate that A. ramidus was definitely bipedal. Individuals were small and are described as resembling chimpanzees in appearance. A. ramidus fossils are associated with evidence of a woodland habitat – questioning the hypothesis that bipedalism evolved when our ancient ancestors moved into the open habitat of the savannah. Sahelanthropus remains also come from a wooded habitat.
In March 2005 scientists published the description of a 3.8mya fossilfound in the Afar region of Ethiopia (Gibbons, 2005). This skeleton includes bones from the pelvis, legs and arms, spine, and pectoral girdle, and was clearly that of a bipedal hominin. While it has yet to be named, its finders say that it is quite distinct from the older A. ramidus.
There is some suggestion (e.g. Gee, 2001) that bipedalism is a symplesiomorphy for humans; an ancestral trait shared with other related taxa. If Orrorin turns out to be outside the hominin line, then either bipedalism evolved more than once (an example of convergent evolution), or it evolved before the last common ancestor of apes and humans. Going on the palaeobotanical, isotopic, and ecological evidence, both Orrorin and Ardepithecus lived in woodlands – the habitats of modern chimpanzees and gorillas. Were they in fact early chimps, rather than ancient hominins? There is certainly a tendency to assign all finds of this age, no matter how ambiguous, to our lineage. Perhaps one, or even both, does belong to the ancestral chimpanzees.
This confirmation that bipedal locomotion evolved very early in our history fits well with our current understanding that evolution of bipedalism preceded any marked increase in cranial capacity. In the 1920s there would have been quite a different interpretation placed on these remains, in the light of the prevailing belief that the large brain that sets Homo sapiens apart from all other primates evolved before a fully erect bipedal stance. This allowed the Piltdown fraud to go undetected for so long, despite the clear (in hindsight) evidence that the remains were doctored. It also influenced the reception that Raymond Dart received on presenting the Taung baby (Australopithecus africanus) to the scientific community.
At the same time that fossil and molecular data have improved our understanding of timing and events at the beginning of our lineage, they have also pushed back in time the first appearance of anatomically modern H. sapiens. Mitochondrial and nuclear DNA studies suggested that modern humans had their origins in Africa 150 – 200,000 years ago (the “African Eve” hypothesis & “Out of Africa” hypotheses) Then in 2003 three modern sapiens skulls were found in Herto, Ethiopia and dated at 160,000 – 170,000 years old. This year that date was extended even deeper in time by new studies of the stratigraphy associated with the Omo 1 & 2 fossils, also from Ethiopia. These were discovered in 1967 but their actual age was unclear. However Ar40/ Ar39 ratios from Feldspar in the relevant rock strata give an age of 195±5 thousand years, making these the earliest anatomically modern humans described to date (McDougal et al., 2005).
The Herto skulls belonged to two adults and a juvenile. All appeared to have been defleshed after death. Sediments associated with the fossils contained butchered remains of hippopotamus and bovids, together with stone tools that had a combination of Middle Stone Age and late Acheulean features. (Stringer 2003).
Both these finds provide strong evidence for the “Out of Africa” hypothesis, which has anatomically modern humans evolving – like all other hominins – in Africa then migrating outwards into the Middle east, Europe, Asia and Australia. Here they replaced (or interbred with, in some accounts) the older hominins from earlier migrations out of Africa. The pattern of migration is still unclear , although Chris Stringer (2000) suggests a coastal pathway. This is supported by mtDNA studies published May 2005. Amongst other things , this hypothesis explains why Australia was settled so much earlier than Europe, given the earlier model of an exodus via the Red Sea region directly into the Middle/near East & Europe.
Fossil evidence tells us that sapiens shared at least Europe and the Middle East with Homo neanderthalensis until that species became extinct around 25-30,000 years ago. From then on ours seemed to be the sole remaining twig on the once bushy human family tree. In November 2004 the announcement of another new fossil find, this time from Indonesia, caused a revision of this view – and generated an on-going controversy in palaeoanthropological circles.
The “hobbits” – as they were dubbed by the popular press – were found in a cave on the Indonesian Island of Flores, which gave rise to their scientific name of Homo floresiensis (Brown et al., 2005; Gibbon, 2004). Stone tools, similar to those produced by Homo erectus, had been found in the region some years before. The fossils represent 6-7 individuals, one of which – a female – was relatively complete and included a well-preserved skull and jaw. She was short, at only 1 m in height, and small-brained (380cc), with limb proportions reminiscent of australopiths. Her encephalisation quotient (a measure of the ratio of body mass to brain size) was in the range of 2.5 – 4.6, compared to 5.8 – 8.1 for H. sapiens, 3.3 – 4.4 for H. erectus, and 3.6 – 4.3 for H. habilis. Facial and dental features and the post-cranial anatomy argued for the fossil’s inclusion in the genus Homo, while features of the cranium, together with the EQ, suggest a relationship with H. erectus. More recent (April 2005) data on virtual comparisons of endocasts suggest that the floresiensis brain is more similar to that of erectus than of other hominins.
The research team decided that the phylogenetic history of floresiensis indicated that it was a dwarfed descendant of erectus, which had colonised the island at least 500,000 years ago. Endemic dwarfing is relatively common in island species: the small size of this species is an adaptation to life on an isolated island with a low calorie diet and a lack of large predators. Excitingly, the remains are only 12-18,000 years old – suggesting that sapiens and floresiensis (and before it, erectus) may have cohabited in this region until quite recently.
The conclusion that this represents a dwarfed descendent of erectus is not universally accepted. There was considerable alarm in the scientific community when the remains were sequestered by Professor Jacob, an eminent Indonesian palaeonanthropologist, who was not part of the team that discovered the bones. He announced his belief that the bones were in fact the remains of a microcephalic modern H. sapiens and a descendent of the local population of erectus. This rather heated debate may reflect the fact that the protagonists have different interpretations of the origins of modern humans: the original research team take the out-of-africa approach, while professor Jacob and his supporters incline to the Multiregional hypothesis developed by Milton Wolpoff.