Cambrian

but there are indirect traces of life in the shape of limestone probably derived from shells and of black lead probably derived from plants all these early deposits have been terribly twisted and contorted by subsequent convulsions of the earth and most of them have been melted down by volcanic action so that we can tell very little about their original state thus the history of life opens for us like most other histories with a period of uncertainty its origin is lost in the distant vistas of time Still, we know that there was such an early period, and from the thickness of the rocks which represent it, we may conjecture that it spread over three out of the ten great aeons into which I have roughly divided geological time.

Next comes the period known as the Cambrian, and to it we may similarly assign about two and a half aeons on like grounds.

The Cambrian epoch begins with a fair sprinkling of the lower animals and plants, presumably developed during the preceding age, but it shows no remains of fish or any other vertebrates.

To the Silurian, Devonian, and Carboniferous periods, we may roughly allow an aeon and a fraction each, while to the whole group of secondary and tertiary strata, comprising almost all the best-known English formations, Red Marl, Lias, Oolite, Greensand, Chalk, Eocene, Miocene, Pliocene, and Drift, we can only give a single aeon to be divided between them.

Such facts will sufficiently suggest how comparatively modern are all these rocks when viewed by the light of an absolute chronology.

This will show that there was plenty of time for their development in the earlier ages.

True, we must reckon the interval between ourselves and the date of this blue mud at many millions of years.

But then we must reckon the interval between the Elias and the earliest Cambrian strata at some six times as much, and between Elias and the lowest Laurentian beds at nearly ten times as much.

Then there's the simple fact that extinct species are always found in the same rock layers you'd expect to find them.

Which is why you don't see a bunny skeleton in Cambrian rock layers from half a billion years ago.

That's also how we know that Dimetrodon is not a dinosaur.

Closely related species are often geographically distributed near one another.

So after Snowball Earth, the Ediacaran era gives us the first extensive fossil evidence for multicellular organisms.

There were various ancient forms that resemble today's worms, corals, mollusks, various underwater plants.

But then in the Cambrian era, adaptive radiation really got underway and multicellular life filled thousands upon thousands of niches, unlocked by their new traits.

A lot more is just possible for multicelled organisms than for single-celled ones.