Dr Alison Campbell
Department of Biological Sciences
School of Science and Technology
The University of Waikato
Private bag 3105
Hamilton, New Zealand.
Phone +64 7 838 4732
E-mail acampbell@waikato.ac.nz
www.waikato.ac.nz
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.