The “Cambrian explosion” marked the rapid appearance of many animal phyla that persist today, and began about 570 million years ago (mya). Life itself appeared in the fossil record as simple cyanobacteria—”blue green algae”—about 3.6 billion years ago (bya); the first “true” cell with a nucleus probably arose about 2 bya; and the first multicellular organism between 1 and 2 bya.
The “explosion”, contrary to some creationists, wasn’t instantaneous, so it couldn’t have marked a single creation “event” at one time. Rather, the origination of many (but not all) modern phyla occurred between 570 and 540 mya. So the “explosion” took at least thirty million years.
Still, the reason why life lingered so long in a rather simple form, and then rapidly diversified into multifarious and complex forms, has been a long-standing puzzle in paleobiology. As I said, it can’t be due to an instantaneous creation, but intelligent-design advocate Stephen Meyer has a new book coming out that will claim that this rapid appearance reflects the action of
the Christian God an unknown intelligent designer. It’s likely to float yet another god-of-the gaps argument, but I’ll leave the assessment of its validity to the professional paleontologists who will undoubtedly review Meyer’s book. (Note to the DI, which loves to quote-mine me: my characterization of the Cambrian explosion as a “puzzle” does not mean that I think a naturalistic explanation will always elude our understanding, or that I give IDiots any credibility in your previous explanations involving a designer.)
Anyway, real scientists are at work on the problem, and have proposed many non-goddy solutions, including the accumulation of sufficient oxygen to allow the diversification of complex life, the advent of “toolkit genes”, like Hox genes, that could be coopted to build different body plans, the availability of empty niche space after the Ediacaran fauna went extinct, “arms races” between newly evolved predators and prey that drove things like the development of armor, and so on. Wikipedia has a list of competing explanations, and of course some could have acted in concert.
And new explanations continue to arise. One, proposed by geologists Robert Gaines and Shanan Peters, was just published as a short note in New Scientist, which, sadly, is behind a paywall. But I can summarize it briefly.
Being geologists, Gaines’ and Peters’ (G&P’s) hypothesis is geological, and rests on the observation that the geological strata reveal a huge section of missing rock, called the “Great Unconformity,” that may represent a billion lost years of Earth’s history.
G&P suggest that it is the weathering of this crystalline rock layer that gave the impetus to the Cambrian explosion. This erosion is likely to have filled the oceans with mienerals: calcium, magnesium, silicon dioxide, phosphates, and bicarbonates.
Eventually the accumulation of these minerals in the ocean might have permitted the already-present but simple life forms to cross the threshold of biomineralization: the incorporation of environmental elements into hard parts like shells, exoskeletons, bones, and teeth. It’s costly to make these parts, since it involves the use of metabolic pathways that can divert energy from reproduction, and perhaps the absence of minerals before the Great Weathering would have prohibited the evolution of hard parts. But the sudden influx of these compounds could have made the evolution of biochemical pathways for mineralization feasible, and their acquisition advantageous. Hard parts, like shells and exoskeletons, are useful in many ways: protecting you from the environment or predators, providing support, allowing larger body size, and so on.
As the authors note,
We suspect that the elevated concentration of ions in seawater effectively lowered the evolutionary barrier for biomineralisation. Today, most organisms invest energy in creating biominerals because hard body parts are so ecologically and evolutionarily advantageous. But evolution couldn’t “forsesee” how useful biominerals would be when shape into the teeth, claws, and shells we know today. [JAC: I'm not clear why the preceding sentence is in there. Clearly hard parts couldn't evolve before the requisite minerals were available.] Instead, we think the ion influx promoted by the last stages in the formation of the Great Unconformity may have lowered the energy barrier to biomineralisation or caused biominerals to appear as metabolic byproducts. The usefulness of these new raw materials meant that natural selection could quickly take over.
Well, there you have it. I’m not a paleobiologist, so I can’t judge whether this explanation is a good one, or is contradicted by other observations. Professionals in the area will undoubtedly weigh in. But one thing is for sure: if we’re going to have a satisfying explanation of the Cambrian explosion, it’s going to be a scientific one—one that is testable and does not invoke the action of
the Christian God an intelligent designer. Saying that “an intelligent designer did it” is the equivalent of saying “Sam did it.” It’s a non-answer, and yet another attempt to plug the gaps in our knowledge with God an intelligent designer.