About 540 million years ago the Pre-Cambrian Ediacarans were succeeded by the creatures of the Cambrian era. For the first ten million years, these beasties were also pretty weird, leaving behind fragments of spines and spikes which presumably are the remains of prototype skeletons that hadn't yet joined up. At that point, nature suddenly learned how to do joined-up skeletons, and much else: this was the time known as the Cambrian Explosion. Twenty million years later virtually every body-plan found in modern animals was already in existence: everything afterwards was mere tinkering. The real innovation of the Cambrian Explosion, though, was less obvious than joined-up skeletons or tusks or shells or limbs. It was a new kind of body plan. Diploblasts were overtaken by triploblasts ...
Sorry, Archchancellor. We mean that creatures began to put another layer between themselves and the universe. Ediacarans and modern jellyfish are diploblasts, two-layered creatures. They have an inside and an outside, like a thick paper bag. Three-layered creatures like us and practically everything else around are called triploblasts. We have an inner, an outer, and a within.
The within was the big leap forward, or at least the big slither. Within you can put the things you need to protect, like internal organs. In one sense, you are not part of the environment any more, there is a you as well. And, like someone who now has a piece of property of their very own, you can begin to make improvements. This is a lie-to-children, but as lies go it is a good one. Triploblasts played a crucial role in evolution, precisely because they did have internal organs, and in particular they could ingest food and excrete it. Their excreta became a major resource for other creatures; to get an interestingly complicated world, it is vitally important that shit happens.
But where did 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 volcanic vents. Or hot rocks deep underground. Or they could be seashores. Imagine layers of complicated (because that's easy) but disorganized (ditto) molecular gunge on rocks which are wetted by the tides and irradiated by the sun. Anything in there that happens to produce a tiny 'space elevator' establishes a new baseline for further change. For example, photosynthesis is a space elevator in this sense. Once some bit of gunge has got it, that gunge can make use of the sun's energy instead of its own, churning out sugars in a steady stream. So perhaps 'the' origin of life was a whole series of tiny 'space elevators' that led, step by step, to organized but ever more complex chemistry.
UNNATURAL SELECTION
THE LIBRARIAN KNUCKLED SWIFTLY through the outer regions of the University's library, although terms like 'outer' were hardly relevant in a library so deeply immersed in L-space.
It is known that knowledge is power, and power is energy, and energy is matter, and matter is mass, and therefore large accumulations of knowledge distort time and space. This is why all bookshops look alike, and why all second-hand bookshops seem so much bigger on the inside, and why all libraries, everywhere, are connected. Only the innermost circle of librarians know this, and take care to guard the secret. Civilization would not survive for long if it was generally known that a wrong turn in the stacks would lead into the Library of Alexandria just as the invaders were looking for the matches, or that a tiny patch of floor in the reference section is shared with the library in Braseneck College where Dr Whitbury proved that gods cannot possibly exist, just before that rather unfortunate thunderstorm.