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A NEW analysis of the fossil record casts doubt on the long-held assumption
that biodiversity has been steadily rising for millions of years. If the
findings are right, they could sink one of the major theories in the field of
palaeobiology.

Until now, most researchers accepted that biodiversity mushroomed after a
mass extinction 250 million years ago. But a new database of marine fossils
suggests that levels of biodiversity have actually stayed largely unchanged
since that time. The implications are huge, says John Alroy at the University of
California at Santa Barbara.

鈥淥ur results, if correct, could have big implications on how we understand
evolution,鈥� Alroy says. If biodiversity is static, he says, then it鈥攁nd
hence evolution鈥攎ust be regulated more by predation and competition
between species than by ecological factors such as climate.

The evidence for an explosion in biodiversity comes from another database of
30,000 marine fossils set up in the 1970s by palaeobiologist Jack Sepkoski, then
at the University of Chicago. To cope with the limited computing power available
at the time, Sepkoski simplified the data by recording only the dates of a
species鈥� appearance in the fossil record without noting population sizes.

Such short cuts created substantial biases. For example, the apparent
explosion in diversity 250 million years ago could be an artefact of the sheer
number of fossils recovered from that period.

To address these problems, a consortium of palaeobiologists led by Alroy and
Charles Marshall at Harvard University revamped the database by compiling data
on thousands of fossilised clams, molluscs and other marine invertebrates. They
divided the prehistoric record into discrete 10-million-year time slots and used
statistical sampling to gauge the number of species in each slot.

They looked at two time periods. The first, 460 to 300 million years ago,
included the Palaeozoic 鈥減lateau鈥�, when biodiversity supposedly stayed constant
after the Cambrian explosion 540 million years ago. The other was the late
Jurassic to early Palaeogene period, 164 to 24 million years ago, when accepted
theories say species diversity ballooned.

Using the revised database, the researchers applied eight sampling methods
aimed at reducing bias. Overall they confirmed that biodiversity stayed constant
or even decreased during the Palaeozoic plateau. Results varied depending on the
sampling method used, but in one model, the late Jurassic explosion vanished
altogether. The researchers saw at most only a small rise in biodiversity during
the Jurassic to Palaeogene period.

The database covers only marine fossils from the northern hemisphere, but the
researchers plan to fill in the gaps. They will analyse the Cambrian explosion
next.

More work needs to be done. But 鈥渋f these results stand up to further
analysis, they will quite substantially change our picture of the pattern of
biodiversity鈥�, says Mark Newman, a palaeontologist at the Santa Fe Institute in
New Mexico.