Fewer notes on this chapter, with is primarily about further exploring just how dramatic a change the Cambrian explosion was, as now especially illustrated by a rich fossil find located in the Burgess Shale formation of Canada by paleontologist Charles Walcott in 1909. This is about 27 years after the death of Darwin, to provide some context. The author spends some time sharing some of the remarkable critters found in the formation – buy the book for that!
Especially remarkable, again, is just how many different body forms made their appearance all at once.
During this explosion of fauna, representatives of about twenty of the roughly twenty-seven total phyla in the known fossil record made their first appearance on Earth.
The author spends a good deal of time dwelling on the fact that the fossil record seems to be backwards from what a Darwinian explanation would predict. We would expect a slow accumulation of many small changes, which would eventually produce what we would recognize as new body types. Instead the fossil record seems to suddenly show a whole bunch of new body types, and then over time small changes accumulate within those already-existing types – exactly backwards.
The actual pattern in the fossil record, however, contradicts this expectation. Instead of more and more species eventually leading to more genera, leading to more families, orders, classes, and phyla, the fossil record shows representatives of separate phyla appearing first followed by lower-level diversification on these basic themes.
But, of course Darwin and Walcott were not dumb people and recognized the problem of missing fossils for transition animals – so what is the solution? As we’ve already mentioned, Darwin figured that with time, the missing fossils would be found. Walcott’s discovery made matters worse, but he also proposed the transition fossils would eventually be found, but now with a little more sophistication, proposing a theory for both why they were missing from the Burgess Shale and where they later might be found.
Called the “Artifact Hypothesis”, the idea is roughly as follows. Walcott suggested that the ancestors of the Cambrian animals evolved during a time called the, ahem, Precambrian, when sea levels were lower (and that last part is the critical fact here). At the beginning of the Cambrian period, sea levels rose, depositing all the newly-evolved Cambrian animals on the continents where some of them would (near as I can tell) eventually be fossilized in features like the Burgess Shale. The older transition fossils, then, are just in a different place because the animals lived in a different place (in deeper water), and could today be located in deep-sea sediments, which are obviously more difficult to access. So Walcott also proposed that the missing fossils would eventually be found, but further proposed where you might expect to find them (thus actually providing a testable hypothesis), as soon as sufficient deep-sea drilling techniques were invented.
So… have they now been found? Well that’s the end of the chapter! Though I would say, the author does not drop promising hints.
Walcott’s theoretical accomplishment was no mean feat. His discovery of the Burgess Shale was like a defense attorney with absolute faith in his client stumbling upon a room stuffed with clues that would seem to discredit him.
Ahem. Well, we shall see where the story goes.
Side comment – I will say, reading Darwin’s explanation, you do get the sense that is really is an excellent story. I know many others have said the same. Humans need stories, and you do wonder if Darwin’s ideas would not have caught on with nearly the same force if they didn’t tell such a nice creation story (really). Eh.
See you next chapter.