Travel Reference
In-Depth Information
often been observed to play with fl oating objects in aquaria. And, like us,
octopuses can quickly open a screw-top jar once they are shown the trick.
When, how, and why did the mollusk lineage part company with ours?
We now have some answers to these profound questions.
Comparisons of DNA sequences show clearly that the mollusks and the
chordates had a common ancestor. But when these sequences are compared
base-by-base, it also becomes clear that many changes—single bases and entire
chunks of sequence that have been substituted, inserted, and deleted—have
taken place during our divergence from that common ancestor. The accumu-
lation of these numerous dif erences shows that our common ancestor was
remote from us in time. Indeed, that common ancestor lived so far back in time
that it is also the ancestor of most of the organisms that I found at Lembeh.
This DNA sequence analysis shows that two major diverging branches
of animals arose early from the common ancestor. A sub-branch of one of
these two great branches gave rise to the mollusks, along with other impor-
tant sub-branches that led to the arthropods and to various kinds of worms.
The other major branch led to the vertebrates, including us. This second
branch also gave rise to further sub-branches that led to—among others—
the echinoderms. It is this second major grouping of lineages that sparked
my evolutionary musings as I hovered over that spiny mob of fi re urchins in
the Lembeh Strait. The DNA evidence is unequivocal: the echinoderms are
much more closely related to us than the mollusks or the arthropods.
We can put fi rm dates on some of the events in our ancestry. This is
because, whenever we have both DNA evidence and fossil evidence about
the ancestry of animals, they tend to agree beautifully. For example, there are
a relatively small number of DNA dif erences between ourselves and chim-
panzees—our DNA sequences are 96% identical, and we share almost all our
genes. Such a high level of identity tells us that we do not have to travel very
far back in time to fi nd our common ancestor. The fossil record agrees with
the molecular evidence. Both lines of evidence show that the common ances-
tor of humans and chimpanzees lived about six or seven million years ago.
The estimates agree so well because our own fossil record is so well studied
and because the fossil record of the mammals provides multiple calibration
points for the times at which various mammalian DNA divergences began.
