“SMASHING a paradigm is rejuvenating,” says Phillip Tobias. He should know.
To mark his 70th birthday five years ago, Tobias urged his fellow
palaeoanthropologists to ditch one of the central dogmas of human
evolution—the notion that our ancestors made their first great advance
towards human form by swinging out of the forest and into the open savannah,
where they began walking upright. “Open the window, and throw out the savannah
hypothesis,” was Tobias’s rallying call.
Today, that paradigm has been so thoroughly bashed that some people argue it
never really existed. But Tobias isn’t finished yet. Although physically frail,
when he gets up on the podium he has the delivery and mental agility of a man
half his age. And this giant of palaeoanthropology is once again challenging his
audience. If humanity wasn’t born out of a move into Africa’s hot, open spaces,
then how? “It’s time to open our minds,” says Tobias, a professor at the
University of Witwatersrand in Johannesburg. He wants the academic establishment
to consider the heretical idea that we were born of water.
Forty years ago, Â鶹´«Ă˝ published a feature entitled, “Was
man more aquatic in the past?” (17 March 1960, p 642). In it, Alister Hardy, a
distinguished biological oceanographer and Fellow of the Royal Society, went
public with an idea that he had sat on for almost three decades, fearing it
would jeopardise his career. “My thesis,” he wrote, “is that a branch of the
primitive ape-stock was forced by competition from life in the trees to feed on
the seashores.” Hardy argued that if our ancestors were semi-aquatic, it might
explain major physical differences between humans and the other primates.
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The move into a watery environment would account for our exceptional swimming
abilities and the fact that newborn babies can swim and float, said Hardy. It
would also have put pressure on our ancestors to start walking upright, in order
to keep their heads above water. This in turn freed up their hands, allowing
them to use tools—probably starting simply by cracking open shellfish with
stones, just as California sea otters do today. Hardy also noted that we are the
only “naked” ape, and that loss of hair is a characteristic of some aquatic
mammals. Many such creatures have a layer of fat just beneath the
skin—another feature which distinguishes us from other primates. Our
profusion of sweat glands would then be a counterbalance to this insulating
layer, an adaptation to keep us cool when out of the water.
Maybe Hardy should have trusted his instincts and kept quiet. He had hoped
his idea might be “discussed and tested against further lines of evidence”. What
he got was the cold shoulder. The “aquatic ape” theory never provoked anything
more than derision or embarrassment among anthropologists, and to this day only
one academic exchange on the subject has been published.
Hardy quietly dropped his idea, but it was soon picked up in a surprising
quarter. From a valley in rural south Wales, housewife and dramatist Elaine
Morgan began compiling evidence and writing books in support of the aquatic-ape
theory—the first of which appeared three decades ago. Morgan compared the
anatomy, biochemistry and physiology of modern humans with other animals, and
used these comparisons to extend Hardy’s original arguments. She pointed out,
for example, that with 10 times the fat cells you’d expect for an animal of our
size, we are by far the fattest primates. Our babies are the only ones born fat.
The subcutaneous layer laid down during the last month of gestation grows
thicker during the first months of life. And this is white fat—not great
for insulation but an excellent aid to buoyancy. Unlike the fat of most mammals,
this fat is bonded to the skin, just as it is in dolphins, seals, and
hippos.
Morgan has found several aquatic adaptations to add to Hardy’s list. In
particular, she argued that some of the physical traits that make speech
possible evolved as a result of living in water, rather than to improve
communication skills. Among terrestrial mammals, we alone have voluntary control
of our breathing, an ability shared by all diving mammals. Likewise, no other
land mammal has a descended larynx, which is useful for speech but also allows a
swimmer to gulp large quantities of air quickly through the mouth.
Morgan wasn’t entirely surprised by the reaction she got from the academic
establishment. “They viewed me as a crackpot. `Farcical’ was an average
comment,” she recalls. “Probably the main reason was their strong conviction
that they already had the answer.”
Indeed, all the evidence that emerged during the 1970s and 1980s seemed to
support the savannah theory. Fossils were found in the hot, dry grassland areas
of South Africa and the Rift Valley, bolstering the idea that our ancestors were
“killer apes” who moved into the open to hunt their prey. Water had no role to
play in this view of human evolution. Competition in the harsh, savannah
environment, the theory held, led to an upright gait, tool-use and expanding
brainpower. But no one—including Tobias, once one of the strongest
proponents of this view—stopped to think that areas which are savannah
today may not have been so in the past.
Then, in the 1990s, Tobias and his colleagues finally checked. They examined
fossilised pollen recovered with 2.7-million-year-old hominid remains from a
grassland site at Sterkfontein, 50 kilometres north-east of Johannesburg. The
pollen suggested that the area had been more wooded than was previously
suspected. But the clincher came with the discovery of fossilised
lianas—vines that hang from forest trees. These could not have come from
open savannah.
Other South African hominid sites also yielded plant and animal remains
characteristic of ancient forests. Then came similar findings in Ethiopia, at
the site where the famous “Lucy” was found, and also alongside what is probably
the oldest hominid discovered, dating back more than 4 million years. The
inescapable conclusion was that Ethiopia, too, was heavily wooded when our
ancestors lived there.
But if human evolution wasn’t kick-started by a move to the savannah, what
could account for the emergence of such an unusual animal? Tobias was quick off
the mark in the search for a new answer. He noticed that what all the fossil
hominid sites had in common was proximity to water. Then there was the obvious
fact that our species cannot go long without a drink, and seems to waste large
amounts of fluid in sweat and urine. “If ever our early ancestors were
savannah-dwellers they must have been the most profligate urinators on the
savannah,” he says.
We need a constant supply of water, but was there more to it than that?
Tobias is not yet sure whether water played a decisive role in the initial split
between human and chimp ancestors. But his recent research has convinced him
that by about 2 million years ago, our ancestors were adapted to a coastal
environment, and this is what allowed them to spread across the globe.
The sea level was much lower than it is today when the first hominids moved
out of Africa, Tobias notes. Land that is now submerged would have been exposed,
allowing Homo erectus to walk from Africa around the coast of southern
Asia to Siberia. But it wasn’t just a matter of colonising the beaches. He
points to two particular finds as evidence that these immediate forerunners of
Homo sapiens were not only adapted to life on the coast, but were also
at home in the water.
Just beyond the island of Bali in the Indian Ocean lies a deep underwater
trough. Even with sea levels low, there would still have been at least 19
kilometres of water separating Bali from the next island. Yet recently unearthed
remains of ancestral elephants and stone tools on the island of Flores, well
beyond this barrier, date back around 900,000 years. How did they get there?
It’s likely that the elephants swam. Despite appearances, elephants are strong
swimmers—their record is 48 kilometres—and they can use their trunks
as snorkels. As to how the tools got there, our own ancestors may have floated
or rafted over—this is long before the first boats appear in the
archaeological record. Or they too could have swum, says Tobias.
This second possibility, he says, is bolstered by hominid artefacts found in
south-eastern Spain dating from 1 to 1.5 million years ago. The extreme age of
the remains suggest that Homo erectus took a short cut, swimming across
the five-kilometre-wide Strait of Gibraltar rather than trekking through the
Middle East and across mountainous southern Europe. “Have we been swimmers for
one million years?” asks Tobias. “I believe it’s very likely.”
It’s a conservative estimate, according to Marc Verhaegen from the Centre for
Anthropological Studies in Putte, Belgium. He points to fossil evidence from
Arabia which indicates that an ancestor of all the great apes was living in
watery forest margins 17 million years ago. Verhaegen believes that this animal
would have waded on two legs in the water and moved effortlessly through the
trees, in much the same way as the mangrove-dwelling proboscis monkeys of Borneo
do today. Most primates find it easy to adopt an upright gait when necessary, he
says, because arboreal adaptations have left them with a highly mobile spine and
flexible limb joints.
Verhaegen also cites evidence from hominid teeth to support the idea of a
long association between apes and water. At the Museum of Mankind in Paris,
Pierre-François Puech has been looking at the microscopic features of the
tooth enamel. Teeth from the fossils of both eastern and southern Africa have a
glossy, polished surface typical of animals that feed on succulent marsh and
riverside vegetation.
Our ancient affinity for water can also be seen in the anatomy of our hands,
Verhaegen argues. By walking upright, the ancestral primate would have kept its
hands free for manipulating objects and climbing. This would have allowed the
hands to evolve into supremely dexterous tool-making appendages.
Verhaegen also believes that the knuckle-walking we now see in chimps and
gorillas evolved later, as their forebears moved inland. Support for this idea
comes in the form of the 3.3-million-year-old fossil hand from South Africa
known as Little Foot. Its discoverer, Ron Clarke of the University of Frankfurt,
described it at a meeting early this year as “quite modern” in form. He
concluded that the short palm and fingers suggest that Little Foot did not
knuckle-walk, undermining the old notion that we are descended from primates who
did.
“There’s no doubt that the differences between chimps and humans must be
explained by a waterside past which included wading and diving,” Verhaegen says.
And he sees the repercussions extending right down through the ages to our own
genus. Some Neanderthals had bony outgrowths in their ear canals. In modern
humans, such ossification is only found in lifelong divers, suggesting to
Verhaegen that Homo neanderthalensis spent much time in the water.
Though Verhaegen holds an extreme view in the debate on aquatic origins,
other researchers also see signs that we have been shaped by a watery
environment. Michael Crawford, a biochemist from the Institute of Brain
Chemistry and Human Nutrition at the University of North London and his
colleagues are convinced that without water, Homo sapiens could never
have evolved the trait that above all others makes us human: our big brains.
Fossil skulls reveal that for the first 3 million years after the split with
chimps, our ancestors’ brains barely increased in size. But then, around 2
million years ago, they began to grow steadily. The past 200,000 years show a
sudden, exponential growth, and a 50 per cent increase in cranial capacity from
Homo erectus to Homo sapiens. “As far as the biochemistry is
concerned, I cannot see how a large brain could have evolved on the savannah,”
says Crawford. He points out that the general trend among savannah-dwellers is
towards larger bodies and smaller brains. At the extreme, a 1-tonne rhino has a
350-gram brain—less than 0.1 per cent of its body mass.
Crawford believes that two particular fatty acids hold the key.
Docosahexaenoic acid (DHA) is needed to construct the membranes of neurons and
photoreceptor cells, while arachidonic acid (AA) is a crucial component of the
walls of blood vessels, without which a large brain cannot grow or function.
Crawford, along with Andrew Sinclair from RMIT University in Melbourne and
others, has looked at the chemical composition of brains from 42 different
animal species and found that they all contain the same proportion of DHA and
AA. What’s more, there is no substitute for either of these fatty acids. These
findings have strengthened the researchers’ suspicion that brain size is limited
by the availability of DHA and AA.
Both fatty acids are in extremely short supply and are slow to form within
the body. DHA is especially rare in the diets of large savannah-dwelling
mammals. Although a precursor of AA is found in the seeds of flowering plants,
the only place where DHA is abundant in the food chain is in the world’s oceans,
lakes and rivers. This is where the first primitive nervous system evolved. “DHA
has a 600-million-year track record,” says Crawford.
Marine nutrients are likely also to have hastened the growth of the human
brain, he argues, pointing out that people whose diets are deficient in DHA and
AA suffer mental and vascular illnesses. “No other theory which attempts to
explain the human brain offers any molecular mechanism. But there is a lot of
science in support of the right ecological niche,” says Crawford. It might
explain why chimps and humans are so different despite our remarkable genetic
similarity, he adds.
The earliest fossils of modern humans found so far come from coastal sites on
the Red Sea and the southern cape of South Africa. Domestic remains indicate
that these people were eating seafood 100,000 years ago, at the same time that
the human brain was expanding. Crawford envisages generations of
people—women and children in particular—harvesting and eating this
abundant and reliable food source. This would have directed the essential fatty
acids straight to where they could have the greatest effect. Mother’s pass on
DHA to their babies through the placenta and via breast milk. What’s more,
recent evidence shows that bottle-fed babies who receive DHA and AA supplements
are better at problem solving than those who do not.
“Modern Homo sapiens ate fish and shellfish consistently as a
requirement for brain expansion and increased intelligence,” says Crawford’s
colleague, Leigh Broadhurst from the US Department of Agriculture near
Washington DC. “The diet came first.”
And it wasn’t just on Africa’s coastline. Genetic analysis of people alive
today indicates that the first modern humans arose inland, in the Rift Valley,
another place where water is central to life. With Crawford, Claudio Galli from
the University of Milan, and others, Broadhurst has shown that people living
around Lake Turkana and Lake Nyasa still benefit from a diet rich in DHA and AA.
She also points out that there is no other environment on Earth like this. It is
the only place where tectonic drift started to create an ocean and then the
process stopped in its tracks. Broadhurst believes that our uncommon
intelligence arose out of this unique environment.
So has the scientific community finally accepted the amateur enthusiast,
Morgan? Not quite. Two years ago, anthropologists invited her for the first time
to address a big meeting, but the majority remain sceptical. They are
particularly concerned that most of the support for her ideas comes from the
physiology and form of modern humans, not the fossil record. Even those who like
Morgan’s ideas in principle say that more research is required. Tobias argues
that the theory should be renamed, perhaps as “water and human evolution”. While
he and others are quite satisfied that water played a major role, they suspect
that “aquatic-ape theory” is misleading and will never be taken seriously.
Just how aquatic our ancestors were is a thorny issue. These days, even
Morgan sees no evidence for a fully aquatic hominid. But she and her supporters
can’t agree on how aquatic it actually was. “No one is able to commit to an
answer,” says John Langdon from the University of Indianapolis. And then there’s
the problem of exactly when this association with water occurred. “The
chronology of the aquatic-ape hypothesis does not work,” says Langdon. The
traits Morgan points to as aquatic adaptations appear one by one over a
4-million-year period. Unless humans lived largely in the water for that entire
time, Langdon argues, invoking an aquatic phase to explain our evolution just
complicates the issue.
Chris Stringer from London’s Natural History Museum also downplays the role
of water. He sees coastal living as a late development, “part of the expanding
adaptive horizons of modern humans”. He has recently described a possible
coastal route out of Africa, and is especially impressed by the speed with which
Homo sapiens made the long trek to Australia. New dates indicate they
arrived at least 60,000 years ago, suggesting our seafaring abilities are older
than many anthropologists had suspected.
There are still researchers who greet the new developments with about as much
enthusiasm as Hardy received back in 1960. But even they don’t doubt the
savannah hypothesis is dead. Whether it will be replaced by an amphibious theory
remains to be seen. The tide is turning, though. “The ridicule placed at the
door of the aquatic-ape theory is easy to generate, just as the President of the
Royal Society ridiculed Mr Faraday’s useless experimental demonstrations of
electricity,” says Crawford. “Ridicule is a popular and political tool but not a
scientific tool. If you want to challenge a thesis, you do it with facts and
science.” The aquatic-ape debate is at last being fought on those grounds.