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Thursday, 7 June, 2001, 18:01 GMT 19:01 UK
New evidence in extinction whodunnit
Cave painting of mammoth
Earlier humans knew a variety of larger animals
By BBC News Online's Ivan Noble

Scientific research papers normally make dry reading, but this one reads almost like the start of a whodunnit:

"All Australian land mammals, reptiles and birds weighing more than 100 kilograms perished in the late Quaternary," Richard G Roberts of the University of Melbourne and his colleagues write in the journal Science.

And the question, of course, is indeed: Who did it? Who or what could possibly have caused the extinction of so many different creatures in what was, geologically speaking, a short period of time?

Investigations have been underway for more than a century and two main suspects have emerged.

Two suspects

The killer may have been a change in the climate, possibly the onset of the last ice age. Or it may have been that well-known offender with a very long record, Homo sapiens.

As detective film fans know, the case against the killer often hinges on establishing the time of death, and this time around is no exception.

But for decades it has been difficult to establish when exactly Australia's giants died out. There are limits to the accuracy of radiocarbon dating.

Now Richard Roberts and his colleagues have combined two other dating techniques to come up with more precise timings for a whole range of fossils from Australia and Papua New Guinea, which in times of lower sea levels were joined by land.

Key dates

And it looks like hard work for Homo sapiens' defence team:

Our data are consistent with a human role in extinction.

Richard G Roberts
University of Melbourne
Optical and uranium-thorium dating techniques both indicate that the extinctions are most likely to have taken place around 46,000 years ago.

With the last ice age at 19 to 23,000 years ago, the date is much too early for the climate to have been the culprit.

Instead, the evidence points to human culpability, since the first humans came to today's Australia around 56,000 years ago.

The scientists are not clear about the modus operandi of the killer, however.

Because of the margins of error, they cannot be sure whether humans hunted the giant lizards, birds and mammals to extinction, or whether they simply caused so much disruption to the ecosystem that the extinctions came as a consequence.

Either way, it looks like another case of destruction by humans.

"Our data are consistent with a human role in extinction," the researchers write.

US study

The Australian research is published at the same time and in the same journal as a study using computer predictions to try and discover what was responsible for similar mass extinctions in North America.

John Alroy of the University of California, Santa Barbara, developed a computer prediction taking into account different possible numbers of humans in ancient North America, their hunting ability, the degree of competition between species and the geographic dispersal of different species.

His predictions matched closely to reality, correctly predicting what actually happened to 32 out of 41 prey species.

And not only was he able to predict which species would die out, but also when they would die out.

His conclusion was that the major extinctions in North America occured between 800 and 1,600 years after humans turned up around 13,000 years ago.

The destruction took place on a timescale that was, in geological terms instantaneous, but nevertheless slow enough in terms of human generations for those who unleashed it to be unaware of what they were doing.

Another indictment for Homo sapiens, but material at least for a mitigation plea.

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See also:

22 Apr 01 | Asia-Pacific
Gourmet kangaroos face extinction
12 Sep 00 | Festival of science
Humans 'face extinction'
08 Sep 98 | Sci/Tech
Humans came close to extinction
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