> The Cambrian Explosion is a radiation of multicellur, hard-shelled forms
> from a particular time (approx 550 Myr). The fish appear at the end of the
> Cambrian and radiate from that time. Tetrapods radiate from the Upper
> Devonian. Reptiles from the Pennsylvanian. Mammals radiate from the
> Triassic, Whales radiate from the Eocene. Each of these events for the
> various groups, has the appearance of lineages coming from a single cause.
>
By way of clarification of my earlier response to this argument by Morton:
Perhaps all you say is true. But it applies to the period from the Cambrian
onward, which encompasses only the last 12 per cent of the age of the earth
(if you accept 4.5 billion years). I think Pun's original mention of
organisms like archaebacteria and RNA-based and sulfur-metabolizing life
implied that he was referring to life at earlier times. The rules may have
been quite different, even diverse, at those times. But due to natural
selection, eventually one set of metabolic and reproductive processes
tends to dominate, so by the time you get to the Cambrian, the range of
options could have been reduced to one.
So we need to define our time scale more carefully. Of course, there may
be little or no evidence for these earlier stages, unless one somehow
can establish that the existing archaebacteria are "living fossils" that
go back to these early pre-Cambrian times. Can this be done?
Paul Arveson
arveson@oasys.dt.navy.mil 73367.1236@compuserve.com
Radar Cross Section and Target Physics Branch (Code 724)
Naval Surface Warfare Center, Bethesda, MD 20084-5000
Voice (301) 227-3831 FAX (301) 227-1914