Earth's strange past and the fossil record
A very general conclusion made from the theory of common descent is that life, as a whole, was
different in the past. The predicted evolutionary pattern is that the farther back we look back in
time, the more different life should appear from the modern biosphere. More recent fossils
should be more similar to contemporary life forms than older fossils.
This point is related to, yet subtly different from, prediction 1.4
and prediction 1.5
predicted common ancestors. As we have seen, the standard phylogenetic tree predicts many
common ancestors and their morphologies. However, given what we know of modern species
dynamics and recent extinction rates, we know that the majority of organisms will eventually go
extinct (Diamond 1984a
; Diamond 1984b
; Wilson 1992
, ch. 12; Futuyma 1998
, pp. 722-723). By
extrapolation, the majority of past organisms also have gone extinct. Thus, we should reasonably
expect that the predicted common ancestors had many other descendants and relatives that did
not leave descendants which survive today. In short, we predict that the majority of fossil species
that we find should not be the actual common ancestors of modern species, but rather they
should be related organisms that eventually ended in extinction.
The oldest rocks we find on the earth are about 4 Bya (billion years ago), and they are devoid of
any life. The oldest potential fossil evidence for life are fossil bacteria from the Apex Chert of
Australia (3.46 Bya), though these fossils are currently embroiled controversy and may not
represent traces of life. The next oldest fossils are well-accepted fossil bacteria and bacterial mats
(stromatolites) from South Africa that date to 3.4 Bya. Thus, the oldest fossil prokaryotes date to
3.4 to 3.5 Byr. For nearly the next billion years, rocks from the Archean have no multicellular
life at all, just prokaryotes. The oldest eukaryote fossils are acritarchs dating to about 1.75 Byr.
For another 1000 million years, there is still no evidence of multicellular life.
Near the Precambrian/Cambrian transition, only 580 Mya, in the Ediacaran and Burgess shale
faunas we finally find the first fossils of multicellular animals. However, they are very unusual,
mostly small, soft-bodied metazoans, and most are superficially unlike anything found today.
Precisely as we would expect from the standard phylogenetic tree, the earliest fossils of multi-
cellular life are very simple sponges and sea anemone-like organisms (sea anemones and
jellyfish are both cnidarians). Around 20 million years later, we find the first evidence of simple
mollusks, worms, and echinoderms (organisms similar to starfish and sea cucumbers). Another
~15 million years later, the very first vertebrates appear, though most people would strain to
recognize them as such. They are small worm-like and primitive fish-like organisms, without