Australopithecus afarensis vs. chimps
From: Andrew Nowicki (andrew_at_nospam.com)
Date: 07/29/04
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Date: Thu, 29 Jul 2004 19:35:24 +0200
At first sight it may seem that Australopithecus afarensis was
a sort of degenerate chimp. Its weight was about 25% smaller
than the average weight of the chimp, its brain size was the
same, its hands were very weak, although dexterous, and its legs
were very powerful. Both species could use sticks and stones as
tools. The Australopithecus afarensis fossils were found in open
habitats: dry bushland, riparian woodland, and riverine forest
habitats. Their dentition rules out frequent dining on red, raw
meat or grass. Chimps had much stronger hands and were better
tree climbers, so they could easily shoo the Australopithecus
afarensis interlopers who attempted to feed in a forest occupied
by the chimps. The big question is why the Australopithecus
afarensis survived and thrived despite competition from the
chimps. It seems that the chimps rather than the Australopithecus
afarensis should have dominated the riparian woodland. Apparently
the strong legs of the A. afarensis were more important in the
riparian woodland than the strong hands of the chimps. Someone
may argue that the Australopithecus afarensis had the advantage
because they could outrun crocodiles. This argument is not
convincing because the chimps could escape the crocodile's jaws
by quickly climbing the trees. The strong legs of the A.
afarensis are not suitable for fast running -- almost every
other animal living in its habitat could run faster. These legs
were good only for heavy lifting and standing. It is unlikely that
A. afarensis hauled huge quantities of food. If it did, chimps
would stole the food cache. It is more likely that A. afarensis
hauled heavy stones or heavy branches. I said it before and I
say it again: I believe that A. afarensis made safe ground nests
which enabled a small family to forage safely in the vicinity of
the nest and to sleep safely inside the nest. These nests were
made of sticks and stones and were as strong as beaver's lodges.
Small family size enabled A. afarensis to find enough food
within safe distance of their nest. When a pack of chimps came
to the vicinity of the nest, the chimps could not find enough food
for their relatively large pack and had to live. The chimps were
smart enough to figure out who ate their dinner, but could not
easily evict the A. afarensis interlopers. I do not know if the
chimps would attack A. afarensis hiding inside the nest. A more
likely scenario is that A. afarensis left the nest when the
angry chimps came to evict them. Chimps could damage the nest,
but they were harassed by the relatively fast running A.
afarensis, so they quickly realized that their struggle was
futile.
____________________________________________________________________________
RELATED LITERATURE:
W.-J. Wang and R. H. Crompton, "The role of load-carrying in the evolution
of modern body proportions," J. Anat., 204, pp. 417–430, 2004.
http://www.liv.ac.uk/premog/PDFs/15kjoa.pdf, excerpt:
"The first unquestionably bipedal early human ancestors, the species
Australopithecus afarensis, were markedly different to ourselves in body
proportions, having a long trunk and short legs. Some have argued that
'chimpanzeelike' features such as these suggest a ‘bent-hip, bent-knee’
(BHBK) posture would have been adopted during gait. Computer modelling
studies, however, indicate that this early human ancestor could have
walked in a reasonably efficient upright posture, whereas BHBK posture
would have nearly doubled the mechanical energy cost of locomotion,
as it does the physiological cost of locomotion in ourselves. More
modern body proportions first appear at around 1.8–1.5 Ma, with Homo
ergaster (early African Homo erectus), represented by the Nariokotome
skeleton KNM-WT 15000, in which the legs were considerably longer in
relation to the trunk than they are in human adults, although this
skeleton represents an adolescent. Several authors have suggested that
this morphology would have allowed faster, more endurant walking. But
during the same period, the archaeological record indicates a sharp
rise in distances over which stone tools or raw materials are transported.
Is this coincidental, or can load-carrying also be implicated in selection
for a more modern morphology? Computer simulations of loaded walking,
verified against kinetic data for humans, show that BHBK gait is even
more ineffective while load-carrying. However, walking erect, the
Nariokotome individual could have carried loads of 10–15% body mass for
less cost, relative to body size, than AL 288-1 walking erect but unloaded.
In fact, to the extent that our sample of humans is typical, KNMWT
15000 would have had better mechanical effectiveness in bearing light
loads on the back than modern human adults. Thus, selection for
effectiveness in load-carrying, as well as in endurant walking, is indeed
likely to have been implicated in the evolution of modern body proportions."
K. Steudel and M.C. Tilken, "The energetic costs of the relatively short
legs characteristic of Australopithecus and Neanderthals"
abstract from http://www.pennpress.org/journals/pa/2004PASocietyAbstracts.pdf:
"The consequences of the relatively short legs characteristic of AL 288-1
have been widely discussed, as have the causes and consequences of the short
legs of Neanderthals. Previous studies of the effect of leg length on the
energetic cost of locomotion have reported no relationship, however, limb
length could have accounted for as much as 19% of the variation in cost and
gone undetected (Steudel & Beattie,1995; Steudel, 1994, 1996). Kramer (1999)
and Kramer & Eck (2000) have recently used a theoretical model to predict the
effect of the shorter leg of early hominids, concluding that the shorter
limbs may actually have been energetically advantageous. Here, we took an
experimental approach. Twenty one human subjects, of varying limb lengths,
walked on a treadmill at 2.8 and 3.0 mph, while their expired gases were
collected and analyzed. The subjects walked for 12 minutes at each speed and
their rates of oxygen consumption (VO2) over the last four minutes were
averaged to estimate VO2. We also measured each subject’s height, weight and
leg length. Lean body mass and %fat were determined using dual-energy x-ray
absorptiometry. ANCOVA with total VO2 at either speed as the dependent
variable and total lean mass, % fat and leg length as covariates resulted in
all three covariates having a significant positive effect on VO2 at p < 0.01.
Subjects with relatively longer legs had lower locomotor costs. Thus the
short legs characteristic of some hominid taxa would have resulted in more
costly locomotion. The magnitude of this effect is substantial; Neanderthals
are estimated to have had locomotor costs 30% (or more) than those of
contemporary anatomically modern humans. By contrast the increase in leg
length seen in H. erectus would have mitigated the increase in locomotor costs
produced by the increase in body size."
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