Travel Reference
In-Depth Information
How we and the mollusks fi rst parted company
The DNA evidence makes clear that my feeling of kinship with the mollusks
of Lembeh is well founded. But how and why did we fi rst part company, and
why did we take such separate evolutionary paths? These questions are much
harder to answer, because they require evidence from the fossil record that
we do not yet have.
Some intriguing hints of such evidence come from Precambrian shales
and carbonate rocks in the Doushantuo formation of Guizhou Province in
southern China. These beds, which date from 40 million years before the
Cambrian, have yielded some tiny but well-preserved fossils that look like
dividing cells. They might, as their discoverers suggest, be the remains of
simple one-celled animals with cells like ours.
Tiny vase-shaped fossils from the same beds look as if they might have
been the embryonic stages of small animals. But these embryos, if that is
what they are, are not accompanied by any signs of the animals that they
might have grown into. Nowhere in the Precambrian rocks have scientists
yet discovered anything that looks like the later Cambrian organisms, aside
from fragmented glassy skeletons of early sponges and that enigmatic proto-
mollusk
Kimberella
.
Why were the ancestors of the Cambrian organisms so small and soft-
bodied that they left such sparse and enigmatic fossils? Andrew Knoll sug-
gests that two things may have happened. First, oxygen levels gradually rose
to the point at which larger organisms could have been supported. But at the
outset this rise may have had little ef ect, because the ecological niches of the
Precambrian world were already full and there were simply no opportunities
for such large-bodied creatures. At the beginning of the Cambrian, however,
there is evidence for an extinction event. The resulting wave of extinctions
emptied ecological niches everywhere on the planet. These new opportuni-
ties for life, coupled with the more plentiful oxygen, may have been the trig-
ger for the Cambrian explosion.
If this hypothesis is correct, then the tiny ancestors of mollusks, worms,
arthropods, and chordates that lived at the beginning of the Cambrian and
