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 didnít 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. Dartís 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 Dartís 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 Dartís 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 ~todayís 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) Vrbaís 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 donít 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. McKeeís 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 - canít support or disprove Vrbaís 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 didnít 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 doesnít 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.