166 CHAPTER 7 | The First Bipeds
An objection to the above scenario might be that
when bipedalism developed, savannah was not as exten-
sive in Africa as it is today (Figure 7.14). In both East
and South Africa, environments included closed and
open bush and woodlands. Moreover, fossil flora and
fauna found with Ardipithecus and the possible human
ancestors from the Miocene are typical of a moist, closed,
wooded habitat.
However, the presence of bipedalism in the fossil record
without a savannah environment does not indicate that bi-
pedalism was not adaptive to these conditions. It merely
indicates that bipedalism appeared without any particular
adaptive benefits at first, likely through a random macro-
mutation. Bipedalism provided a body plan preadapted to
the heat stress of the savannah environment.
In an earlier era of human evolutionary studies, larger
brains were thought to have permitted the evolution of bi-
pedalism. Around the mid-20th century, theories for the
adaptability of bipedalism involved a feedback loop be-
tween tool use, brain expansion, and free hands brought
about by bipedalism. We now know not only that bipedal-
ity preceded the evolution of larger brains by several mil-
lion years, but we can also consider the possibility that
bipedalism may have preadapted human ancestors for
brain expansion. According to Wheeler,The brain is one of the most metabolically active
tissues in the body.... In the case of humans it
accounts for something like 20 percent of total
energy consumption. So you’ve got an organ pro-
ducing a lot of heat that you’ve got to dump. Once
we’d become bipedal and naked and achieved
this ability to dump heat, that may have allowed
the expansion of the brain that took place later in
human evolution. It didn’t cause it, but you can’t
have a large brain unless you can cool it.^18or late in the day, or when it is absent altogether at night.
Some, like antelope, are evolved to tolerate high body tem-
peratures that would kill humans due to overheating of the
brain tissue. They accomplish this through cooling their
blood in their muzzles through evaporation before it en-
ters the vessels leading to the delicate tissues of the brain.
According to Wheeler, the interesting thing about hu-
mans and other primates is that
We can’t uncouple brain temperature from the
rest of the body, the way an antelope does, so
we’ve got to prevent any damaging elevations in
body temperature. And of course the problem is
even more acute for an ape, because in general,
the larger and more complex the brain, the more
easily it is damaged. So, there were incredible se-
lective pressures on early hominids favoring adap-
tations that would reduce thermal stress-pressures
that may have favored bipedalism.^17The idea that bipedal posture reduces the amount of
heat to which humans are exposed is not completely new,
but Wheeler has scientifically studied this phenomenon.
He took a systematic series of measurements on the ex-
posure of an early biped, like Lucy, to solar radiation in
upright and quadrupedal stances. He found that the bi-
pedal stance reduced exposure to solar radiation by 60
percent, indicating that a biped would require less water
to stay cool in a savannah environment compared to a
quadruped.
Wheeler further suggests that bipedalism made the
human body hair pattern possible. Fur can keep out solar
radiation as well as retaining heat. A biped, with reduced
exposure to the sun everywhere except the head, would
benefit from hair loss on the body surface to increase the
efficiency of sweating to cool down. On the head, hair
serves as a shield, blocking the solar radiation.
(^17) Ibid. (^18) Ibid.
Late Miocene
through to Pliocene
SAVANNAH AND
WOODLAND DESERT DESERT
DESERT
SAVANNAH
SAVANNAH
Lake
Megachad
SAVANNAH
RAINFOREST
SAVAN DESERT
SAVANNAH N AH
AND
WOODLANDMONTANE
FORESTS
RAINFOREST
REFUGIA
Present
1
2
3
4
5
SHRUB
SAVANNA
H, WOODLAND,
SAVANNAH,
WOODLAND,
SHRUB
Pleistocene
RAINFOR
ES
T
RAINFO
RES
T
PLIOCENE SITES
- Afar
- Lake Turkana
- Lake Baringo
- Olduvai Region
- Transvaal, S. Africa
Figure 7.14 Since the late Miocene, the vegetation zones of Africa have changed considerably.