The Science of Discworld Revised Edition

all those triploblasts come from? Were they an offshoot of the Ediacarans? Or did they come from something else that didn’t leave fossils?
    It’s hard to see how they could have come from Ediacarans. Yes, an extra layer of tissue might have appeared, but as well as that extra layer you need a lot of organization to exploit it. That organization has to come from somewhere. Moreover, there were these occasional tantalizing traces of what might have been pre-Cambrian triploblasts – fossils not of worms, which would have clinched it, but of things that might have been trails made by worms in wet mud.
    And then again, might not.
    In February 1998, we found out.
    The discovery depended upon where – and in this case how – you look for fossils. One way for fossils to form is by petrification. There is a poorly known type of petrification that can happen
very
fast – within a few days. The soft parts of a dead organism are replaced by calcium phosphate. Unfortunately for palaeontologists, this process works only for organisms that are about a tenth of an inch (2 mm) long. Still, some interesting things are that tiny. From about 1975 onwards scientists found wonderfully preserved specimens of tiny ancient arthropods – creatures like centipedes with many segments. In 1994 they found fossilized balls of cells from embryos – early stages in the development of an organism – and it is thought that these come from embryonic triploblasts. However, all of these creatures must have come
after
the Ediacarans. But in 1998 Shuhai Xiao, Yun Zhang, and Andrew Knoll discovered fossilized embryos in Chinese rock that is 570 million years old – smack in the middle of the Ediacaran era. And those embryos were
triploblasts
.
    Forty million years before the Cambrian explosion, there were triploblasts on Earth, living right alongside those enigmatic Ediacarans .
    We
are triploblasts. Somewhere in the pre-Cambrian, surrounded by mouthless, organless Ediacarans, we came into our inheritance.
    It used to be thought that life was a delicate, highly unusual phenomenon: difficult to create, easy to destroy. But everywhere we look on Earth we find living creatures, often in environments that we would have expected to be impossibly hostile. It’s beginning to look as if life is an extremely robust phenomenon, liable to turn up almost anywhere that’s remotely suitable. What is it about life that makes it so
persistent
?
    Earlier we talked about two ways to get off the Earth, a rocket and a space elevator. A rocket is a thing that gets used up, but a space elevator is a process that continues. A space elevator requires a huge initial investment, but once you’ve got it, going up and down is essentially free. A functioning space elevator seems to contradict all the usual rules of economics, which look at individual transactions and try to set a rational price, instead of asking whether the concept of a price might be eliminated altogether. It also seems to contradict the law of conservation of energy, the physicist’s way of saying that you can’t get something for nothing. But, as we’ve seen, you can – by exploiting the new resources that become available once you get your space elevator up and running.
    There is an analogy between space elevators and life. Life seems to contradict the usual rules of chemistry and physics, especially the rule known as the second law of thermodynamics, which says that things can’t spontaneously get more complicated. Life does this because, like the space elevator, it has lifted itself to a new level of operation, where it can gain access to things and processes that were previously out of the question. Reproduction, in particular, is a wonderful method of getting round the difficulties of manufacturing a really complicated thing. Just build one that manufactures more of itself. The first one may be incredibly difficult – but all the rest come with no added effort.
    What is the elevator for life? Let’s try to be general here, and look at the common features of all the different proposals for ‘the’ origin of life. The main one seems to be the novel chemistry that can occur in small volumes adjacent to active surfaces. This is a long way from today’s complex organisms – it’s even a long way from today’s bacteria, which are distinctly more complicated than their ancient predecessors. They have to be, to survive in a more complicated world. Those active surfaces could be in underwater