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Students of anthropology have become accustomed to intellectual eruptions
in the discipline, as newly discovered fossils turn cherished theories on
their heads. One such eruption began a few weeks ago, and a mighty one it
is. This time, however, theories are in ruins not because of new fossil
finds in remote parts of Africa or Asia, but as a result of delicate work
in a high-tech basement laboratory in Berkeley, California.

There, in the Geochronology Center of the Institute of Human Origins,
Garniss Curtis and Carl Swisher subjected a few crystals of volcanic mineral
to the most advanced geological dating technique yet developed. They raced
their results into the 25 February issue of Science, and were im-mediately
besieged by reporters eager to tell the world about the astounding discovery.
Bold headlines appeared in major newspapers around the world, and stories
were told of disbelief at what had emerged from the Berkeley laboratory.
‘Fossils from the dawn of humanity are rewriting the story of evolution,’
exclaimed the cover of Time magazine.

Central to this rewriting is the notion that early humans, Homo erectus,
left Africa almost a million years earlier than current theory allows.
If they did, then the anthropologists face three big challenges to their
view of human prehistory: in the understanding of what kind of animal H.
erectus really was, in unravelling a puzzle of early technological sophistication
and how it spread throughout the Old World, and in helping to resolve that
most contentious issue of current debates, the origin of modern humans.

Extraordinary claims

The conclusion about the earlier-than-expected move out of Africa flows
from Curtis and Swisher’s redating of those few crystals, which they claim
reveal the age of three fossil specimens from the island of Java, in Indonesia.
One of the fossils, the partial cranium of a child, was unearthed almost
60 years ago, at the site of Mojokerto. The other two, discovered four decades
later at Sangiran, are fragments of face and skull. These were widely accepted
as the oldest early humans outside Africa. Most anthropologists believed
the Mojokerto child and the Sangirans had died no more than a million years
ago. Not so, say Curtis and Swisher.

The crystals reveal that the fossils are much older than was believed
– the child died 1.81 million years ago and the Sangiran individual, 1.66
million years ago. ‘This is just overwhelming – no one expected such an
age,’ exclaimed Berkeley anthropologist F. Clark Howell in Time magazine.
There is a maxim that runs, ‘extraordinary claims require extraordinary
evidence’. By any measure, the Javan claims are extraordinary. So, is the
evidence offered by Curtis and Swisher sufficiently extraordinary?

The answer comes in two parts. The first concerns the accuracy of the
age that was derived for the crystals. ‘The dates are fine,’ responds Tim
White, another Berkeley anthropologist. ‘That’s not an issue.’ Bernard Wood,
an anthropologist at the University of Liverpool, agrees: ‘The dates are
good,’ he says. ‘Curtis and Swisher are among the best in the business.’
Curtis is a well respected geochronologist, responsible for major developments
in the field, including the ability to date single crystals. This technique
(see ‘Radiometric dating’) was the key to Curtis and Swisher’s willingness
to attempt to date the Javan fossils, a venture from which many geochronologists
have shied.

So, if the dates are undisputed by the experts, is there any uncertainty
at all in Curtis and Swisher’s extraordinary claim? Yes there is. The problem
lies in inferring the age of fossil remains by dating crystals of rock from
the sites of the fossils discovery. When Curtis and Swisher state in their
Science paper that the crystals come from ‘volcanic units in direct association
with the hominid find sites’, their peers need to be convinced they are
right. While admitting that the dates have to be taken seriously, Chris
Stringer laments that ‘the association (between crystals and fossils) is
not as good as you’d like’. Stringer, an anthropologist at the Natural History
Museum in London, speaks for many in the discipline.

For instance, White warns that ‘you have to be concerned about the dating
of any fossil from Java’. The problem, he explains, is one of being able
to say precisely from where in the ground a fossil was recovered. ‘I’m sceptical
of everything in Java,’ states Milford Wolpoff, an anthropologist at the
University of Michigan. ‘I’m not convinced that it’s possible to relocate
the site where the child’s skull came from. There’s much less of a problem
with Sangiran.’

Anyone who goes fossil hunting in Java faces the same challenge. It
is a volcanic island with complex geology. Any surface that can conceivably
be cultivated is covered with crops. As a result, it is no easy task to
place fossils into a well-defined geological context. In the absence of
obtaining absolute ages by radiometric dating – measuring the slow decay
of radioisotopes in volcanic minerals that lie just above the fossil, for
instance – scholars have to rely on indirect inference from a variety of
clues. These include matching animal fossils at the site with those of
known age from other sites. Even when radiometric dating is possible, it
is not always feasible to pluck suitable minerals from the exact spot from
which the fossil was recovered.

Cranial conundrum

‘The problem is even worse with the Mojokerto cranium,’ says White,
‘because it was discovered sixty years ago, so how can you possibly be
sure you’re sampling minerals from the right site?’ The cranium was found
by a local collector, Tjokrohandojo. However, Swisher attempts to counter
scepticism by pointing out that a good geological map was made of the region
at the time. He first visited the site in September 1992 but points out
that Teuku Jacob – a member of the Indonesian team that found the Sangiran
remains – revisited the site in 1975 with Tjokrohandojo, who showed him
where the skull was discovered. He also says that extensive field work by
a Japa-nese team during the 1980s relocated the site. ‘We can be fairly
confident we were looking in the right place,’ says Swisher.

When it was found, the skull was full of rock matrix. This offers another
means of checking that the correct volcanic ash layer is being sampled.
Volcanic ash from different volcanoes and from different eruptions of the
same volcano have characteristic chemical, textural and mineralogical compositions.
Detailed analysis can therefore produce a profile of any ash layer. ‘We
did a microprobe analysis of hornblende (a volcanic mineral) from inside
the child’s skull and from the pumice we collected at the Mojokerto site,’
explains Swisher. ‘You can’t tell them apart. I know that doesn’t mean I
can prove conclusively that they are from the same volcanic event, but I’d
bet on it.’

That may sound convincing, but there are still ways in which an erroneously
old date might be obtained. ‘What if the skull had fallen into a channel
that had been cut into old pumice?’ asks Frank Brown, a geologist at the
University of Utah. This possibility arises when streams gradually wear
away rock, cutting deeper courses and sinking into ever older rock. A creature
that falls into such a channel is therefore buried in ancient sediment
and not material that is contemporary with the animal’s life. But there
isn’t any evidence for such channels near the Mojokerto site, says Swisher.

Messy geology

Curtis and Swisher are extremely confident that the mineral they analysed
is from the correct geological stratum, not least because it is the only
layer in the area that contains fossils. Nevertheless, he and Curtis anticipated
criticism such as Brown’s, which is why they included the Sangiran fossils
in their redating venture. If these, too, gave an old date, then the case
for an earlier-than-expected departure of H. erectus from Africa would be
strengthened. ‘Making one mistake would be possible,’ explains Curtis, ‘but
making two would be extremely unlikely.’

There is far less uncertainty about where the Sangiran fossils were
found, not least because their discoverer, Jacob, is still around to verify
it. (Jacob was part of Curtis and Swisher’s expedition, and is a coauthor
of the Science paper.) Nevertheless, the geology of the region is particularly
messy, even by Javan standards. No one can be sure from field observations
whether the strata in which the fossils were entombed was part of the Kabuh
Formation, or the much older Pucangan Formation. But in the end it is the
age, not the name, of the stratum that is important – and here, says Swisher,
the dating of the volcanic material is unequivocal.

This recent expedition to Java is Curtis’s second foray into the uncertainties
of the age of the fossils there. A little more than two decades ago he collected
samples from the Mojokerto region, analysed them at his laboratory in Berkeley,
and obtained an age of 1.9 million years. ‘No one was prepared to accept
it at the time,’ remembers Curtis. ‘People were suspicious of the accuracy
of the dating technique, and in any case, they didn’t want to hear of such
an early date from Java.’

Curtis had used a technique that relied on the slow decay of a radioisotope
of potassium to argon. The method requires that the mineral to be dated
contains significant quantities of potassium – some 1 per cent – to be reliable.
The level in the volcanic minerals of Java have only one-fifth of that.
These days, the dating technique method measures two isotopes of argon,
and is applicable to minerals with extremely low levels of potassium. This
is one reason why Curtis and Swisher can be much more confident in their
results. A second is that, by miniaturising the analysis, it is possible
to make measurements on single crystals, which reduces the danger of contamination.

The caution with which Curtis and Swisher have approached the notoriously
thorny problem of Java, and their reputation as technicians, has encouraged
many anthropologists at least to accept the possibility that the new dates
might be valid. Stringer, for instance, says, ‘The dates have to be taken
seriously. And if they are valid, they are extremely important.’

Out of Africa

One reason for this concerns the behaviour of H. erectus, which appears
to have evolved almost 2 million years ago. The oldest complete skull, for
instance, comes from east of Lake Turkana, in northern Kenya, and is dated
at close to 1.8 million years. Based on this, and some other slightly older
fragments, anthropologists believed that H. erectus evolved first in Africa,
hung around there for about 800 000 years, and then started to expand its
range into Eurasia. ‘We were always having to look for excuses for why H.
erectus didn’t move into Asia earlier,’ comments Wood. ‘If the new dates
are correct, we don’t have to do that any more.’

H. erectus was clearly a different kind of animal from earlier hominid
species, both in anatomy and behaviour. It was much more human in stature
and build than earlier species and, judging from the structure of its teeth
and the content of associated archaeological sites, it apparently included
significant amounts of meat in its diet. As Alan Walker and Pat Shipman,
of the Johns Hopkins University, Baltimore, pointed out in 1989, the transition
from a mainly herbivorous diet to one that is more carnivorous has important
ecological implications. A species that makes such a transition must either
become smaller in body size or expand its home range. H. erectus obviously
didn’t opt for the first option – in fact, it is a bigger animal than earlier
hominid species. It therefore must have expanded its home range and, by
extension, its overall geographical range.

Being carnivorous not only demands an expansion of geographical range,
it also facilitates it as its food sources are markedly increased. ‘Meat
is meat,’ says Walker, ‘and so a carnivore can occupy many different ecological
regions, feeding on herbivores that may be adapted to local plant foods.’
Think of other carnivores, he says, such as lions. In historical times their
range stretched from southern Africa through to Asia and Europe. By entering
the carnivore guild, he says, H. erectus had the same opportunity to expand
its range far beyond the region to which its herbivorous ancestors were
restricted. ‘The puzzle was why it waited for so long to exploit this opportunity,’
says Walker.

One common explanation for this apparent time lag was that, initially,
the species lacked the required technological ability to cope with new
environments. The earliest known stone tools appear in the archaeological
record about 2.5 million years ago. These were simple pebble tools and sharp
flakes of stone. Not until 1.4 million years ago did anything more sophisticated
appear, in the form of teardrop-shaped hand axes, known as Acheulian axes.
When H. erectus learnt how to make these tools, some half million years
after the species first evolved, an expansion beyond Africa became possible.
Or so the story went. It tends, however, to overlook the fact that other
species have expanded their ranges back and forth between Africa and Asia
many times in prehistory, without the benefit of sophisticated technology.

The new dates from Java sink that theory, and indicate that H. erectus
was sufficiently equipped in its anatomy and behavioural attributes to
move beyond the traditional hominid range. Walker points out that this geographical
expansion should not be thought of as a migration, which implies a purposeful
journey: ‘They didn’t pack red spotted handker-chiefs and set out for territories
new,’ he quips. ‘You have to think of it as a gradual spread as the total
population grew.’ A simple calculation shows that, even with a modest territorial
expansion of 10 kilometres per generation, H. erectus populations could
have covered the distance from East Africa to Java in 25 000 years. Slow
by modern standards, perhaps, but lightning quick in terms of archaeology.

A second area where the new dates have an important impact concerns
a long-standing puzzle of the history of H. erectus in the Old World. The
Acheulian tools, the oldest of which come from a site in Ethiopia, are found
in younger sites in western Asia and Europe, but not in eastern Asia, despite
the presence of H. erectus there. Under the old scenario, in which H. erectus
arrived in Asia around a million years ago, the absence of Acheulian tools
has attracted several explanations. Among these is the idea that the ability
to make the tools was somehow forgotten during the population expansion
to the east. A second is that no suitable raw material was available in
eastern Asia from which to manufacture these implements. Another is that
comparable tools could be fashioned from even better material such as bamboo,
which would then break down.

Asian origins?

If, as the new Javan dates suggest, Homo erectus spread into eastern
Asia close to 2 million years ago, the absence of Acheulian technology is
self-evident: the industry had not yet been developed when the first populations
entered Asia. Later populations, now equipped with the know-ledge for making
the new technology, were unable to penetrate eastern Asia, perhaps because
the earlier colonisers were well established and excluded further invasions.
‘It makes for a plausible explanation,’ suggests Robert Foley, an anthropologist
at the University of Cambridge.

This scenario, however, still clings to the notion that H. erectus
evolved in Africa and then spread to the rest of the Old World. A more radical
view held by some anthropologists is that the new dates from Java force
an alternative scenario – that H. erectus evolved in Asia, and then moved
into Africa. The age of H. erectus in those two continents is virtually
the same, so either scenario could, in principle, be correct. One problem
though, as Walker points out, is that ‘there are no known antecedents of
H. erectus in Asia’. While cautioning that the evolution of H. erectus
is by no means clear, Stringer accepts that ‘an African origin has to be
the null hypothesis’.

With the origin of early humans in question, what of the origin of modern
humans? This is the third issue upon which the new dates bear significantly.
The pages of newspapers and magazines these past few years have crackled
with sharp debate between two opposing camps. Protagonists of the ‘out of
Africa’ hypothesis hold that modern humans evolved in a discrete population,
recently, and in Africa. From there, populations spread throughout the rest
of the Old World, replacing existing communities of more primitive forebears.
Protagonists of the multiregional evolution hypothesis contend, by contrast,
that the only spread out of Africa was that of H. erectus, and that modern
humans evolved gradually and more or less in concert throughout the Old
World, wherever populations of H. erectus had become established.

If the new dates are correct, the intellectual gap between the two hypotheses
is widened. ‘The out of Africa hypothesis isn’t affected in any way,’ says
Stringer, the most visible proponent of that camp. ‘It merely clarifies
human history prior to the emergence of modern humans in Africa.’ Of the
multiregional evolution hypothesis, however, he says, ‘It makes it even
less plausible than it was previously.’ Walker is more blunt still: ‘The
multiregional hypothesis is dead.’

Ripples of change

The reasoning is simple. The multiregional hypothesis demands that there
is genetic continuity, or gene flow, throughout all Old World populations
of H. erectus, so that evolutionary change in one region is able to ripple
through other regions. Many geneticists question whether such continuity
is possible over such vast distances, let alone over a million years, as
the old theory required. If H. erectus expanded from Africa into the rest
of the Old World soon after it evolved, this continuity has to be extended
for a further million years, that is from a common stock in Africa all
populations of H. erectus throughout the Old World had then to evolve into
modern humans 2 million years after going their separate ways. ‘That pushes
it beyond the bounds of possibility,’ says Foley.

Wolpoff, the chief proponent of the multiregional hypothesis, dismisses
the notion that the new Java dates are a fatal blow, but concedes that they
‘affect it in an important way.’ Wolpoff argues that anthropologists should
think of multiple dispersals out of Africa, over long periods of time. ‘The
most successful expansions were made possible by the development of Acheulian
industry,’ he argues. ‘It gave the people a competitive edge over populations
that were established from earlier expansions.’ This implies that all populations
established earlier, such as those in Java, were supplanted by the new invaders.’
Yes, these earlier-established populations might have been replaced by
the Acheulian people,’ concedes Wolpoff. However, these assertions sit uneasily
with previous claims that certain anatomical characteristics of the Sangiran
fossils persist in the Australasian region through to recent times.

The new Javan dates clearly have an important impact on the issue of
the origin of modern humans, even if, as Ian Tattersall says, ‘it won’t
affect the beliefs of the converted’. The balance of opinion, however, is
that the multiregional evolution hypothesis has suffered a serious setback.

It’s obvious that the fates of many hypotheses are riding on the validity
of the new dates. They are welcomed by most scholars but anthropologists
would like to be certain before throwing out one basketful of old hypotheses
and setting up new ones. As a result, there are likely to be renewed efforts
to find and date more fossils in Java, to test the validity of the findings
from Mojokerto and Sangiran. ‘If this is all that Garniss and Carl’s work
does, it will have been extremely important,’ observes White.

* * *

Radiometric dating

Many rocks, including volcanic minerals, contain traces of potassium.
One of the isotopes of potassium, potassium-40, decays slowly and regularly
to produce argon-40, an inert gas, thus offering a potential radiometric
clock to measure the passage of time. The more argon-40 a rock contains,
the older it is.

Volcanic rock offers a convenient way of setting the clock to zero,
because all gases, including argon-40, are driven out of the rock crystals
in the heat of the initial volcanic eruption. Anthropologists, therefore,
love to find fossils that have a layer of volcanic ash deposited just a
little higher in the geological strata, because this gives a minimum age
for the fossil.

When potassium/argon dating was first exploited by geochronologists
in the 1950s, they had to measure the amounts of potassium and argon-40
from two separate rock samples. In the 1960s, a new approach was developed,
pioneered by Craig Merrihue at the University of California, Berkeley. He
suggested that the two measurements could be made simultaneously on one
sample, thereby reducing the likelihood of errors.

Naturally occurring potassium contains a large and constant proportion
of a second isotope, potassium-39, which under normal circumstances is stable.
Irradiated with neutrons, it becomes argon-39, cousin of argon-40, which
is then effectively a vicarious measure of potassium in a rock. When the
irradiated rock sample is subjected to high temperatures, both forms of
argon are released and can be determined simultaneously in a mass spectrometer.
It is, however, a laborious method.

During the 1970s and 1980s, a combination of brilliant insight and good
fortune led to a revolution in this form of dating. The conventional heating
method was replaced by a continuous laser beam, and the development of
super-sensitive mass spectrometers enabled tiny samples to be analysed.
These advances were pushed by Derek York and his colleagues at the University
of Toronto and by Curtis and his colleagues. Their work allowed single
crystals to be dated, increasing precision and reducing contamination.

Contamination, the bane of geochron-ology, is caused because a piece
of pumice may contain particles of several different ages. The usual result
of contamination is an artificially old age, as the presence of rock older
than the date of the volcanic eruption contributes disproportionally more
argon-40 to the count. ‘By using single crystals, the contamination problem
is greatly reduced, as a stray, older crystal can immediately be identified,’
says Curtis. ‘It has greatly boosted the power of the technique in anthropology.’