Paleoenvironmental Studies in Africa During the
Existence of Australopithecus africanus
Jeff
McKee (Ohio State University, Columbus, Ohio, USA)
7 October 1997
McKee
has done much work in South Africa.
Taung skull -
discovered in 1924, and brought to Raymond Dart. Dart didn’t visit the
site until many years later. It is the holotype of Australopithecus
africanus, an early hominid. McKee was there to find more hominids,
because debate has occurred about whether the holotype is the same species as
fossils that are called adult africanus. There are some
morphological details that suggest the Taung skull is a robust
australopithecine. Some morphological details also conform with specimens
considered to be adult africanus, but these are ambiguous. The
idea that the Taung skull is a robust australopithecine is mostly based on the
late date of the Taung skull, instead of morphology. The Taung locality
is not well dated, though.
The
Taung skull was the first evidence that human ancestry derived from Africa,
instead of Asia, Europe, Britain, or America. The skull was found near
the edge of the Kalahari Desert, in a limestone quarry of calcareous tufa
deposits. The quarried-out lime was used for gold processing and
agricultural & construction purposes. The skull was blasted out of
limestone tufa - breccia and fossils make up the deposit as well. These
are accretionary deposits. Caves form as accretionary deposits built up
from dolomite weathering in a riverine setting.
Raymond
Dart is credited for finding the skull, but it was brought to him. Dart
identified it as a child, and as an upright bipedal animal, based principally
on the position of the foramen magnum.
Dart
assumed the Taung depositional environment was the same as it is today.
Came up with the savannah hypothesis for hominid origins. Hominids were
adapted to savannahs (treeless plains) from the forested environment by
standing up. This idea is not now believed to be very good. Dart’s
idea was followed until relatively recently.
1988
- work at Taung began. The site had never been excavated properly.
It had been mostly quarried. Some minor excavation work was done in the
1940s. McKee and company are looking for more hominid fossils. Now,
hundreds of africanus fossils are known from South Africa.
At
Taung, found mostly baboon species - two extinct species - they are small,
distant relatives of a modern baboon species. Radiometric dating is not
possible on these rocks. Faunal dating is the only current option.
Faunal dating indicates ~2.5 m.y., which is toward the end of the temporal
range of africanus. Other animals were also found - a buck
(grazer) and rodents (counterparts to modern dry adaptive rodents).
At
2.5 m.y., the environment around Taung was very dry, like Dart’s
savannah. Dart was right, but for the wrong reasons - the environment had
changed since then and back again.
McKee
worked the Taung site for 7 years and never found another hominid fossil.
He was forced to focus on the fossil fauna - evidence for a savannah, but there
are still doubts about Dart’s hypothesis.
Taung
is in north-central South Africa. Most South African hominid sites are
cave sites.
Makaspansgat,
which is in northwestern South Africa, is a dolomite cave site. Has a
huge interior. The value of the site was discovered from quarrying
activity. Was used as a lime source, from the speleothems, the
stalactites, the stalagmites, etc. They left the breccias. There
are 30 hominid fossils from here, and we know much about hominid
morphology. The site dates to 3.0-3.2 m.y., making it the oldest Australopithecus
site in South Africa. The hominid morphology is upright and
bipedal. The modern surrounding environment is different from that seen
at Taung - a heavy dense forest outside the cave in the valley. Lots of
modern animals - lots of primates, cats (leopards). Lots of biodiversity
in the fossils - lots of large mammals (greater diversity than today).
This is indicative of a lush environment. Lots of fossil monkey diversity
(vs. the two species of fossil baboons at Taung). Get five species of
baboons and lots of other things at Makaspansgat. So, Australopithecus
was living in a very different environment than at Taung.
Most
of the South African sites with Australopithecus are forested
environments - hominids are not adapted to the savannah, but hominids were
adaptable, and preferred the forests. This realization was the beginning
of the death of the savannah hypothesis.
Bipedalism
originated in the forests, with the earliest known hominid, Ardipithecus
ramidus (4.5 m.y.). This fossil is scrappy, but appears to be in a
forested environment.
Other
early hominids are found in riverine forest environments (Australopithecus
anamensis at 4.25 m.y. and Australopithecus afarensis at 3.75-2.75
m.y.). The preferred environment of these forms was the forest.
Get
a spurt of first appearances of hominids in the fossil record between 3 and 2
m.y. This timing correlates with an environmental change from the d18O
curve, among other things. Most of Pliocene saw ~today’s
temperatures. From ~3 m.y. to ~2.5 m.y., temperatures dropped, and
started to get cyclical temperature variations on a 4100 year cycle. At
0.9 m.y., temperature dropped again, and got new cycles that appeared at a
frequency of ~100,000 years. This 0.9 m.y. change is the same time when
archaic Homo sapiens first appears.
Elizabeth
(Liz) Vrba’s turnover pulse hypothesis (= immigration, emigration, speciation,
extinction) is based on the above interpreted environmental changes. The
changes correlate with African megavertebrate faunal changes. According
to Vrba and the hypothesis, the environmental change from 3.2 to 2.5 m.y. was
necessary for hominid evolution to occur like it did.
Can
see South African faunal peaks (FADs (first appearance datum) and LADs (last
appearance datum)) connected with each shift in temperature curve.
Are
the FAD and LAD peaks an artifact of the fossil record?
Computer
simulation by McKee - suggests no environment-controlling shift was
present. Distribution of the peaks is solely due to artifacts of
fossilization and distribution of sites, number of species present at each
site, etc.
Did
East African sites as well - FAD and LAD curves don’t show three nice peaks the
way the South African fossil data does. No Vrba-type turnover pulse
occurring here, apparently. She used only bobbets to construct the
hypothesis. McKee’s analysis used all mammals. Computer simulation
mimics the FAD/LAD curve for East Africa as well. Even varied and tweaked
computer simulation parameters (rates of evolution designated to double at
certain points, etc.). Conclusion - can’t support or disprove Vrba’s
hypothesis. No positive evidence for turnover pulses.
The
LAD curve (East Africa) seems to show a higher extinction rate at certain
points than expected from computer simulation. Therefore, overall
biodiversity has decreased since 4-3 m.y.
Hominid
evolution was not controlled by environmental change - hominids had a
preadaptation. Hominids were adaptive organisms. The environmental
change did influence, but didn’t control, evolutionary course.
Vicariance
model of evolution - climate change leads to a shrinkage in established
populations, leading to fragmentation and isolation of subpopulations, leading
to speciation.
McKee doesn’t accept that
the Taung skull was an object of predation by a bird of prey (eagle). Two
baboon skulls are know that are suggestive of eagle predation. It is
actually difficult to discern the pattern of eagle talons make vs. the pattern
leopard teeth marks make. The position of marks happens to be similar.