The Science of Discworld Revised Edition

stopping … and one tall tree made it right up to the present day. Or maybe we’ve reconstructed it incorrectly, amalgamating several different trees into one.
    This new image changes our view of human evolution. One animal in the Burgess Shale, named
Pikaia
, is a chordate. This is the group that evolved into all of today’s animals that have a spinal cord, including fishes, amphibians, reptiles, birds, and mammals.
Pikaia
is our distant ancestor. Another creature in the Burgess Shale,
Nectocaris
, has an arthropod-like front end but a chordate back, and it has left no surviving progeny. Yet they both shared the same environment, and neither is more obviously ‘fit’ to survive than the other. Indeed, if one had been less evolutionarily fit, it would almost certainly have died out long before the fossils were formed . So what determined which branch survived and which didn’t? Gould’s suggestion was:
chance
.
    The Burgess Shale formed on a major geological boundary: at the end of the Precambrian era and the start of the Palaeozoic. The early part of the Palaeozoic is known as the Cambrian period, and it is a time of enormous biological diversity – the ‘Cambrian explosion’. The Earth’s creatures were recovering from the mass extinction of the Ediacarans, and evolution took the opportunity to play new games, because for a while it didn’t matter much if it played them badly. The ‘selection pressure’ on new body-plans was small because life hadn’t fully recovered from the big die-back. In these circumstances, said Gould, what survives and what does not is mostly a matter of luck – mudslide or no mudslide, dry climate or wet. If you were to re-run evolution past this point, it’s quite likely that totally different organisms would survive, different branches of the Tree of Life would be snipped off.
    Second time round, it could easily be
our
branch that got pruned.
    This vision of evolution as a ‘contingent’ process, one with a lot of random chance involved, has a certain appeal. It is a very strong way to make the point that humans are
not
the pinnacle of creation,
not
the purpose of the whole enterprise. 3 How could we be, if a few random glitches could have swept us from the board altogether? However, Gould rather overplayed his hand (and he backed off a bit in subsequent writings). One minor problem is that more recent reconstructions of the Burgess Shale beasts suggests that their diversity may have been somewhat overrated – though they were still very diverse.
    But the main hole in the argument is convergence. Evolution settles on solutions to problems of survival, and often the range of solutions is small. Our present world is littered with examples of ‘convergent evolution’, in which creatures have very similar forms but very different evolutionary histories. The shark and the dolphin, for instance, have the same streamlined shape, pointed snout, and triangular dorsal fin. But the shark is a fish and the dolphin is a mammal.
    We can divide features of organisms into two broad classes: universals and parochials. Universals are general solutions to survival problems – methods that are widely applicable and which evolved independently on several occasions. Wings, for instance, are universals for flight: they evolved separately in insects, birds, bats, even flying fish. Parochials happen by accident, and there’s no reason for them to be repeated. Our foodway crosses our airway, leading to lots of coughs and splutters when ‘something goes down the wrong way’. This isn’t a universal: we have it because it so happened that our distant ancestor who first crawled out of the ocean had it. It’s not even a terribly sensible arrangement – it just works well enough for its flaws not to count against us when combined with everything else that makes us human. Its deficiencies were tolerated from the first fish-out-of-water, through amphibians and dinosaurs, to modern birds, and from amphibians through mammal-like reptiles to mammals like us. Because evolution can’t easily ‘un-evolve’ major features of body-plans, we’re stuck with it.
    If our distant ancestors had got themselves killed off by accident, would anything like us still be around? It seems very unlikely that creatures exactly like us would have turned up, because a lot of what makes us tick is parochials. But intelligence looks like a clear case of a universal – cephalopods evolved intelligence independently