Norm
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DID CHIMP AND GORILLA FOREBEARS WALK ON TWO LEGS?
November 24, 1998
We humans are very peculiar primates. Anatomically we differ from our
cousins the chimps and gorillas much more than they differ from each
other. Yet our forebears must have evolved within the same biological
constraints as all other mammals. So it seems likely that they went
through a series of adaptations that led to all these unique traits and
behaviors we display. Still, this doesn't tell us how and why we got to
be the way we are.
To help answer these questions, we've found it useful to compare human
and hominid anatomy and physiology with other species. Thus, we've
undertaken a systematic study of the parallel or convergent adaptations
of different animals in similar environments. And we find that this
comparative methodology provides us with new ways to interpret the
available evidence. All this has helped us develop some new insights
that we think may shed more light on the evolutionary history of our
human forebears.
As a result of these insights, we now propose that the totality of the
available evidence suggests that the last common ancestor of the
gorillas, chimpanzees and humans may have been forest-dwellers. Even
more startling, it suggests that more than six million years ago, they
may have walked part of the time on their hind limbs. Still, the
question remains, why would this last common hominid ancestor start to
walk on two legs?
THE WADING HYPOTHESIS. Most primates are four-legged tree-dwellers with
mobile joints and limbs. Thus, they can stretch and straighten their
limbs to reach, climb and leap through the trees. Thanks to this
locomotor flexibility, they can also adopt a bipedal gait by extending
their knees and hips. In fact, most primates will adopt this bipedal
gait when they wade through water. They seem to prefer this "linear
stature" any time they must ford rivers or wade through swamps.
As an example of this, the western lowland gorillas are now and then
seen wading on their hind limbs through forest swamps. They do this
when they want to supplement their mostly fruit and Terrestrial
Herbaceous Vegetation (THV) diet with what researchers call Aquatic
Herbaceous Vegetation (AHV). (photo)
In addition, the mangrove-dwelling proboscis monkeys must also cross
stretches of water to move from one mangrove tree to another. Not
surprisingly, they also walk on two legs when making these treks. They
are sometimes even seen using this remarkably human-like bipedal
locomotion on dry ground.=20
But note that this bipedal wading gait differs from the hopping
bipedalism that some primates use when moving on the ground. This
latter gait incorporates bent knees and hips rather than the linear
stature preferred for wading. The advantage of the erect wading posture
is that it allows them to hold their bodies, arms and heads as far as
possible above the water surface. It also allows them to cross through
deeper stretches of water. =20
Rightly or wrongly, most anthropologists still base their estimates of
when human bipedalism started on the available fossil evidence. And up
until a few years ago, they concluded from this sparse evidence that it
started some four million years ago in a savannah environment. But
recent studies and fossil finds such as Ardipithecus ramidus and
Australopithecus anamensis changed their minds. Thus, most
anthropologists now accept that bipedalism started earlier in wooded
and/or forested habitats. =20
All this suggests that the early hominids could have started walking on
two legs part time in a milieu where there was a combination of trees
and water. They might have adopted the typical bipedal wading gait in
seasonally or tidally flooded areas such as a gallery, swamp or mangrove
forest. Note that we think that while the last common hominid ancestor
was a regular wader, it probably still sought refuge, gathered fruits
and slept in the trees. =20
In particular, we now think that several independent yet overlapping
areas of research now suggest an early association with mangrove
forests. This research includes comparative brain size studies, tooth
enamel thickness and wear pattern analyses, mammalian tool use along
with comparisons of the diet and the geographical distribution of living
and fossil ape species. =20
GEOGRAPHICAL DISTRIBUTION OF FOSSIL APES. The hominid group was
formerly believed to include only humans and their supposed fossil
relatives the australopithecines. But DNA comparisons of various ape
species changed many anthropologists' views about our "hominid" family
tree. =20
Anthropologists used to define the chimpanzees and gorillas as
"pongids". But now they usually include them with humans in the hominid
group. That leaves the Asian orang-utans and their presumed fossil
relatives such as Sivapithecus and Gigantopithecus as the only pongids.=20
(See the NS of 29 March, 1998, p 18, Human origins thrown into doubt.)=20
This bio-molecular data also suggests that the pongids (orang-utans) and
the hominids (humans, chimps and gorillas) broke into two groups some
twelve to ten million years ago. Then the forebears of the gorillas and
those of humans and chimps split apart between eight and six million
years ago. Finally, between six and four million years ago, the
ancestors of the chimpanzees and humans became two separate species.=20
(Insert Time line graph.)
Today, the hylobatids (gibbons and siamangs) as well as the pongids
(orang-utans) all come from Asia. On the other hand, the living
hominids (humans, chimpanzees and gorillas) all come from Africa.
But about ten million years ago, the geographic distribution of the
fossil great apes was different. The extinct species that looked most
like the living great apes, such as Dryopithecus, Sivapithecus,
Ouranopithecus and Ankarapithecus, all lived in Eurasia.
This data suggests that pongids and hominids split somewhere in Eurasia
before the hominids entered Africa. So it seemed feasible that an early
hominid population could have clustered in mangrove forests between
Eurasia and Africa near what eventually became the Mediterranean Sea
and/or the Red Sea.
ANCESTRAL DIET. Apes and monkeys lost the ability to produce their own
vitamin C. Most apes primarily eat fruit which is very rich in vitamin
C. Yet most fruits are seasonal and lack proteins and other nutrients.=20
As a result, primates must get their protein in other plant foods or in
animal foods such as insects, meat, fish or shellfish. But since most
animal food sources don't have any vitamin C, they were probably only a
part of our ancestors' diet. =20
Marcus Fernandes of the Em=EDlio Goeldi Museum, Brazil, describes how the
capuchin monkeys that live today in the mangrove areas supplement their
frugivorous diet with oysters. These bivalves are fixed to the mangrove
trunks and are accessible at low tide.
So a dexterous ancestral frugivore that lived in the mangrove forests
could have eaten shellfish as a welcome dietary supplement. Thus, we
began to think it plausible that the last common ancestor of the chimps,
gorillas and humans could have become a part time bipedal wader that
sometimes fed on shellfish.
It is also worth mentioning that Michael Crawford, Stephen Cunnane, Lee
Broadhurst and others connected with the Institute of Brain Chemistry
and Human Nutrition, University of North London, have shown that the
long-chain poly-unsaturated lipid ratios of tropical fish and shellfish
are more like those in the mammalian brain than any other known food
source. So if our forebears had access to this high-caloric and highly
nutritious diet, it could have helped them build and fuel a more
powerful brain.
TOOTH ENAMEL STUDIES. In the 1980s, developmental mechanisms that could
help account for variations in the thickness of the tooth enamel in
mammals and especially in primates were proposed by Laurence Martin of
University College, London. His comparative studies of enamel formation
in apes and humans suggested that most early hominids had thick enamel.=20
The fossil record tends to support this view.
Thick enamel is seen in most of our fossil ape relatives including the
ancestral pongids and hominids. This feature generally holds true
whether they lived before the human-chimp split (for instance, Siva-,
Ourano- and Ankarapithecus, though not in Dryopithecus) or after that
time (the Australopithecus species, though not in Ardipithecus). Today,
we humans and the orangutans still have relatively thick enamel on our
teeth.=20
In living mammals, thick enamel is typical of species that eat
hard-shelled foods like nuts or molluscs. This includes capuchin
monkeys and sea otters. It is interesting to note that Alan Walker
writes: "If, for example, a mammalogist who knows nothing about hominids
were asked which mammalian molar most resembled those of
Australopithecus, the answer would probably be the molars of the sea
otter (Enhydra lutris). This species possesses small anterior teeth, and
large, broad, flat molars with thick enamel." He adds that in the sea
otters, the thick enamel is perhaps not for cracking the shells, but for
the occasional hard inclusions that could otherwise damage the
dentition.
All students of fossil hominid teeth agree that such broad back teeth
with thick enamel and rounded cusps must be linked to the processing of
hard foods. However, in the African apes some of these features may
have undergone an evolutionary reversal. This would help explain why
the mainly herbivorous gorillas and even the predominantly frugivorous
chimpanzees show more reduction in enamel thickness than orang-utans,
who consume more hard-shelled nuts.
Some ten years ago, electron microscopic comparisons by Pierre-Fran=E7ois
Puech and co-workers of the University of Marseille, France, showed that
the early australopithecine molar teeth exhibited extensive enamel
microwear features. These included such characteristic features as a
polished surface with a glossy appearance. This polishing resembles
that seen on the mountain beavers' teeth. This same polishing effect is
evident on rodents such as the capybaras that also feed on marsh plants.
WETLANDS HABITATS. At first, Puech was puzzled by these results because
it was then generally believed that human ancestors were savannah
dwellers. But more recently, this and other evidence suggests that most
if not all hominids probably dwelt in what some refer to as "wet" rather
than "dry" habitats. =20
For example, in 1992, Radosevich and co-workers, in a paper on the
Australopithecus afarensis fossils from Hadar, East Africa, wrote: "The
bones were found in swale-like features ... it is very likely that they
died and partially rotted at or very near this site ... this group of
hominids was buried in streamside gallery woodland."
Along these lines, Rayner and co-workers, wrote in 1993 that the
Australopithecus africanus fossils of Makapansgat, South Africa, were
found in "... very different conditions from those prevailing today.=20
Higher rainfall, fertile, alkaline soils and moderate relief supported
significant patches of sub-tropical forest and thick bush, rather than
savannah ... sub-tropical forest was the hominins' preferred habitat
rather than grassland or bushveld, and the adaptation of these animals
was therefore fitted to a forest habitat."
Since then, other palaeo-environmental reconstructions have confirmed
that early australopithecines typically dwelt in swampy woodlands or
stream side forests. This is where they might have frequently waded
bipedally in search of aquatic plants or certain molluscs. =20
We think that the australopithecines' bipedal build suggests that some
if not all of them spent much of their time wading through shallow
swamps of the tropical forest clearings. Today, the western lowland
gorillas still occasionally wade like this in similar surroundings
looking for starchy sedges and protein-rich herbs. (See the above
photo.)=20
Yet present day chimpanzees and gorillas exhibit few traces of
bipedalism. Other features like extensive tool use, dexterous hands,
slightly larger brains and thick enamel may also have diminished or
disappeared. Perhaps this happened because their ancestors had such an
early interlude of frequent yet part time wading for shellfish or
aquatic vegetation (AHV). In some ways, we can think of some of these
reversals of traits and behaviors as a U-turn back to the more ancestral
lifestyle of other primates. =20
MISSING FOSSIL LINKS. Both the DNA evidence and the fossil record are
incomplete. Still, together, they suggest that some hominids began
living in Africa between eight and six million years ago. Then they
radiated into diverse branches including the australopithecines, the
African apes, our human ancestors and perhaps even other side-branches
that became extinct. Yet the relationships between all these various
branches remain very uncertain. =20
Geologists tell us that fossilization is especially difficult in
mangrove areas. For one thing, tidal water movements can spread the
bones over a vast area. For another, the high acidity may dissolve the
bony remains. What's more, in the mangrove areas the sea floor is flat,
so there is almost no chance that a landslide would cover any remains.=20
This means that the mangrove dwelling hominids were less likely to leave
any fossilized remains that might be subsequently discovered.
Conversely, the inland hominid offshoots that dwelt by lagoons,
estuaries and rivers would be more likely to have left fossil remains.=20
Paleontologists now generally accept the late Colin Patterson's view
that the direct ancestors of some living species may never be found in
the fossil record. Thus it is possible that most or all fossil
Australopithecus and Homo species found to date are simply early or more
recent extinct side-branches of the present living hominids. =20
In part, it was this likelihood that the fossil record might contain
major gaps that led us to be extremely cautious about using it as the
sole basis for attempting to develop viable evolutionary hominid time
lines. So we adopted the practice of assembling and considering all of
the credible available evidence in a comparative and systematic
methodology. While the totality of all the evidence is still quite
spotty and incomplete, the multiple cross checking process does produce
a cautious confidence in the tentative scenarios it suggests. =20
MIGRATION PATTERNS. As noted, the inconclusive available DNA evidence
suggests that some hominids probably entered Africa before eight to six
million years ago. It also indicates that during this same time span,
the gorilla branch and the human/chimp branch divided into two species.=20
By comparing the DNA data and evidence, we can infer that this split
probably took place after their common ancestor entered Africa. =20
We then began to speculate that while the human/chimp branch stayed
along the seacoast, the gorilla branch probably moved inland.=20
Presumably, they went up the rivers to colonize the African interior.=20
So it seemed reasonable to guess that eventually, some of these inland
apes might have evolved into the present-day gorillas.
As mentioned, these herbivorous apes probably fed mainly on Aquatic and
Terrestrial Herbaceous Vegetation (AHV and THV). But as they moved
upstream, they must have consumed less AHV and more THV. Today,
according to recent studies of some lowland gorilla groups, less than
two percent of their diet consists of AHV.
One or two million years after the gorillas' forebears moved inland, the
chimpanzee branch began to follow. They apparently developed parallel
features, but generally remained more omni-frugivorous and spent more
time in the trees. =20
It seems probable that many of the more inland hominid branches would
have evolved in parallel. As they followed the rivers upstream,
shellfish became rarer. So the shellfish part of their diet was
gradually replaced by other plant and animal food. Hence, step by step,
they probably became more herbivorous and spent less time in the water.=20
But note again that while the gorillas' forebears spent most of their
time on the ground, the chimps' forebears retained a more arboreal life
style. =20
COMPARATIVE TOOL USE. Tool use is seen in a long and diverse list of
animals. But perhaps the best mammal tool users are the capuchin
monkeys, the chimpanzees and the sea otters. They all try to open
hard-shelled foods by hammering with hard objects. =20
Sea otters, for instance, crack open shellfish with stones while
floating on their backs. Capuchins crack open nuts with stones. And
Fernandes reports that the mangrove capuchins even use oyster shells
where stones are not available.
We observe many mammals regularly using hard objects as tools to help
them eat hard to get at foods such as this. And we can tell from the
African hominids' thick tooth enamel that they also consumed these hard
foods. Thus, like the capuchins and the sea otters, we would be
surprised if our forebears did not make frequent use of stones for
opening all kinds of hard shelled nuts, and shellfish. Eventually, they
also used rocks to crack open bones for the nutritious marrow.=20
Over time, it is reasonable to assume that our forebears improved on
their bone, nut and shellfish cracking and collecting techniques. Thus,
we can surmise that these omnivores increased the use of stones to help
feed themselves.=20
Then they may have found ways to shape these stones and other tools to
help them accomplish this task. Perhaps this then led to tool use for
other purposes. Arguably, this was the beginning of the human Stone Age
technology. =20
SWIMMING AND DIVING ADAPTATIONS. Today, breath-hold diving is practiced
by some subsistence human cultures that gather shellfish. Some of these
diving capabilities can be attributed to special features in the human
physiology. These aquatic or marine adaptations and changes are obvious
when we compare our features with nonhuman primates. =20
This has been demonstrated by Erika Schagatay, in the physiological lab
of the University of Lund, Sweden. She compared our human diving
response with that of other mammals. So it seems likely that our
forebears' semi-aquatic life style also shaped up their swimming and
diving capabilities and adaptations.
All diving mammals have the ability to take a deep breath at will
whenever they intend to dive. Many of them, like dolphins and seals,
also have large brains. In fact, their brains are much larger than most
land mammals of equal size. =20
Note that many primates like gibbons and other arboreal animals have an
aptitude for vocal and musical expression. They share this capacity for
making and interpreting a wide range of sounds with many marine mammals.
We think that a talent for this kind of complex vocal exchange in
combination with voluntary breathing and large brains, may be some of
the prerequisites for what we now call human language.
OUR COASTAL FOREBEARS. In 1992, M. P. Singh of the Postgraduate
Institute of Medical Education and Research, in Chandigarh, India found
what he believes may be the world's most ancient Homo fossil. In the
Tatrot Formation located in the Siwalik Hills of India, he found an
erectus-like femur and a mandible amidst a number of stone tools.=20
These finds were located in Middle Pliocene beds that were estimated to
be 3.4 million years old. The lower jaw bone featured molar teeth with
crowns in the form of rounded or conical cusps. The thigh bone was
flattened and the marrow was confined to a comparatively narrow strip.=20
The stone tools were apparently shaped for some chopper-like use. =20
Our comparative studies lead us to hypothesize that these attributes of
Singh's finds could be typical of what may turn out to be an early Homo
that spent a lot of time diving for shellfish. So we think it is
plausible that while the ancestors of the African apes and the
australopithecines were dwelling in the interior wetlands of Africa, our
human ancestors were left behind at the coasts. =20
The totality of all the available evidence leads us to propose that Homo
erectus-like people began colonizing the Indian Ocean shores soon after
the human/chimp split some six to four million years ago. There is
even evidence that by two million years ago, some of them reached as far
as Java. If all this proves to be correct, it means that the last
common ancestor of Homo and chimpanzees was even more like Homo than
anyone ever imagined. =20
HOMO INLAND MIGRATIONS. On their way back from being part time swimmers
and waders to becoming full time land walkers, our ancestors, at first,
might have spent a lot of time wading and gathering food in and near the
rivers and lakes. To different degrees, other Homo species may have
preceded sapiens in re-conquering the land as long-legged bipeds. These
include rudolfensis and ergaster in Africa along with erectus and
neanderthalensis in Europe and Asia.
Initially, their bipedality probably took more energy than the
four-legged locomotion of other terrestrial mammals. And it certainly
was a lot slower. But the two-legged walking allowed them to make full
use of their dexterous hands for defense, hunting, tool making, shelter
building and food gathering. It also enabled them to carry weapons,
supplies and their babies over long distances. =20
Was it these dexterous hands, larger brains and spoken language that
then enabled them to venture even farther inland? Was this why they
began to follow in the footsteps of the gorillas and chimps and occupy
the inland milieus along the rivers? =20
Yet in many ways, our ancestors' linear build was better suited for
wading, swimming and diving. Their unique upright stature featured
habitually extended knees and hips. It also included fully stretched
out backs. This latter adaptation is the so-called lumbar lordosis.=20
But when they became more terrestrial again, these aquatic adaptations
also made them prone to lots of back, joint and other ailments.=20
As they went back ashore again, these unusual adaptations probably also
made it too difficult for them to re-adopt to a four-legged gait.
Perhaps this is part of the reason they continued to walk upright on two
legs rather than go back on all fours as the chimps' and gorillas'
forebears had done. =20
HOMO SAPIENS. Today, we are the only mammal left that normally strides
along on two legs. Unfortunately, we also still suffer from frequent
backaches, hip and knee problems. A number of these and other medical
problems are now associated with our bipedal gait and vertical stature.
H. J. Deacon, archaeologist at Stellenbosch University in South Africa=20
describes the archaic Acheulian populations of the African Old Stone Age
as "stenotopic". This means they still occupied a narrow niche in
riverine and wetland habitats. He also says that it was only in the
Middle and Late Stone Ages that modern human populations became
"eurytopic", occupying the same niche that the traditional foragers in
Africa still occupy today. =20
In the past few years, we have been blessed with several new forms of
credible scientific evidence. In addition, new data processing systems
provide us with a number of new ways of compiling and comparing this
ever expanding data base. Yet, when we take all this available evidence
a whole, it is now more obvious than ever how incomplete it is. Still,
when we compare and cross check what is available, we can begin to
decipher a broad outline of what may have happened to our forebears over
the past several million years. =20
This hypothesis is detailed enough that it can be tested against new
evidence as it becomes available. It can also be used as a predictive
tool. As such, its success or failure will then either confirm or
negate its value as the basis for a potentially definitive theory of our
human evolutionary history and linage. =20
The hypothesis states that last common ancestors of the African hominids
were bipedal waders. It says that they were also omnivores that
supplemented their mainly herbi- frugivorous diet with shellfish and
other marine animals. Then it claims that this stem species remained
near the African seacoast while the forerunners of the gorillas and the
chimpanzees migrated inland up the rivers and streams. =20
The hypothesis then tracks this stem hominid through a series of
adaptations to a coastal marine environment. It holds that these
adaptations moved step by step through a series of changes in anatomy
and physiology. It predicts a gradual succession of adaptations from
short legged bipedal part time waders through long legged coastal and
riverine hunter/gatherers.=20
Then it predicts a series of swimming and diving adaptations to extend
their gathering range. The hypothesis also anticipates a more advanced
use of stone tools for cracking open the shells of an ever widening
variety of coastal marine creatures and mollusks. =20
The hypothesis also anticipates a long list of radical anatomical and
physiological changes. These include stripping off all of their fur and
most of the hair from their skin. =20
Then this amphibious life style portends dramatic changes in their heat
retention and cooling systems. This varied life style also transformed
their respiratory system in ways that are unique amongst mammals. From
their palate and their descended larynx to their ability to control
their breathing, these adaptations took them further and further from
the standard primate models. =20
If the hypothesis is correct, we can anticipate that there will be more
proto-Homo fossil finds like that of M. P. Singh. Perhaps they will be
scattered along the Indian Ocean shores from Africa to Java. If our
evolutionary time line hypothesis is sound, these fossils will help us
track the step by step adaptations from part time wader to long legged
Homo-like diver. Or there may be new and better ways to track these
changes through DNA comparisons. =20
We are aware that there is a long list of specialists who have something
to add to the past ten million years of our history. We're also aware
that most of these scientists, academics and interested lay people will
profess to see the fatal flaws in this hypothesis.=20
We readily admit that due to the incomplete evidence it is not difficult
to speculate that it may prove to be off the mark. But to those who are
so inclined, we issue this challenge: Propose another feasible and
parsimonious hypothesis that takes into account all the available
evidence and still purports to explain our unique human anatomical and
physiological differences.