Stephen Jay Gould, the great American paleontologist, liked to say - particularly in A Wonderful Life, that if the tape of evolution were replayed a million times, a species like ours would not necessarily evolve. A debate rages to this day about whether Gould meant chance, as Daniel Dennett claims, or contingency, as Michael Shermer claims.
Biochemist Michael Denton of the University of Otago in New Zealand has an interesting take on the more critical question of what would happen in any case, in Nature's Destiny. He writes,
Curiously, the evolution tape has been played again, at least in part. This has occurred on several occasions. Citing the fact that South America and Australia were isolated from the other continents for the better part of 60 million years, he discusses the marsupials (mammals like kangaroos that nurture their young in a pouch on the abdomen rather than a uterus inside the female's body):
The diversification of the marsupials in Australia is very instructive. Almost every type of placental mammal has its counterpart among the marsupials. There is a marsupial lion, cat, wolf, mole, anteater, jerboa, and flying squirrel. There was even a giant wombat equivalent to the placental rhino. (Denton, Nature's Destiny, 287) He concedes the uniqueness of the kangaroo (a giant jumping rat), but notes that
The skull of the marsupial wolf is amazingly similar to that of the placental wolf.
, a fact discussed and illustrated in The Design of Life (115-16). He notes a similar pattern in South America, and says, "parallelism and long-term evolutionary trends have always struck many biologists, especially paleontologists as difficult to account for in terms of undirected Darwinian models of evolution." (Denton, 288) Denton is not a design theorist, but more of a determinist - that is, he thinks that some events occur in nature because they are constrained by mechanisms that are not yet known. Would the marsupials ever have produced creatures equivalent to modern humans? That's hard to know, because modern humans are very recent in the history of Earth (200 000 years ago at best) and we spread around the globe, including Australia.
It seems like the marsupials never got the chance to try. |