Readers following the cat evolution posts might wonder why we’ve stayed in the Precambrian for so long.
There are three main reasons for that.
Reason #1: Where’d those cats come from?
If felines show up on your doorstep a few minutes before noon and they’re not yours, you’ll need to trace those adorable little paw prints back through the morning to see where they came from.
Paleontology works much the same way, only with fossils and a host of lab studies in addition to the tracks.
This is a stock image, not hard science, but animals — the yellow line — probably did start during the Precambrian, and as you can see from the gray line, the microbes that both animals and plants evolved from go way back. (Arndt and Nisbet; Gradstein et al.; Knoll, 2014; Mukherjee et al.) Image by Dimitrios Karamitros/Shutterstock.
On this “clock” version of the geological time scale, our doorstep is the Phanerozoic (“recent life,” though it goes back some 540 million years to the Cambrian “explosion” of life) — the blue segment.
The Precambrian morning is everything else — a train of multi-billion-year-old individual eons:
Alex Berger, CC BY-NC 2.0.
- Hadean (in red on the graphic).
- Archean (“beginning,” purple).
- Proterozoic (“earlier life,” green).
Three. Eons.
It’s hard enough to grasp how long geological periods are (roughly 80 million years for the dinosaur-packed Cretaceous), or even epochs like the Holocene, just 12,000 years old, in which we’re living now.
Homotherium was around until 11,000 years ago. (Image: James St. John, CC BY 2.0)
The Holocene began when the last ice age ended, and doesn’t that seem like an unfathomably long time ago!
How can we wrap our minds around three Precambrian eons?
More to the point, why should we try?
Because of —
Reason #2: It’s fun!
Well, it is for me — like exploring a new planet and rediscovering your own at the same time — and I’m trying my best to pass that fun along to you.
Precambrian Earth was a happening place!
Supercontinents formed, broke apart, and re-formed; oceans and shallow seas appeared and disappeared, changing color along the way depending on their iron, sulfur, and (eventually) oxygen content; enormous eruptions occurred (this Sunday Morning Volcano series is about some of them); mountain chains rose, weathered down into plains, and sometimes rose anew; global and local climates rang the changes as Earth alternately sweltered or froze.
And life soldiered on through it all, evolving into animals that left their first unambiguous fossils at the dawn of the Phanerozoic roughly 540 million years ago.
That’s amazing! But how did it all happen? Would animals have appeared without all that geological drama?
I don’t know. Even the experts still debate such questions.
But one thing is for sure: it’s interesting.
Something El Albani et al. found in Africa (figures 4 and 7): Are these the fossilized remains of 2.1-billion-year-old life, caught in an offshore Proterozoic mudslide or something else? It matters because they might represent a crucial step in the evolution of complex life like cats: multicellularity. But that hasn’t been established beyond question, yet. (El Albani et al.; Javaux and Lepot)
I’d like to talk about that geology a bit, particularly supercontinents and the intense volcanism associated with them, while carrying on with cat evolution in the main series.
This is both for background and because it’s an opportunity to look at massive eruptions called large igneous provinces (LIPs), which is always fun (since these 100,000-plus km3 events are now long ago and far away).
Reason 3: LIPs affect evolution in major ways
The Precambrian isn’t even on this map! (Image: EreborMountain/Shutterstock)
Our own eon has been exciting, and here’s something you might not know about it: the Phanerozoic is divided into periods that end in life crises because these stand out so clearly in the fossil record (which is pretty good for the last 540 million years).
For instance, in North America, the record shows that nonavian dinosaurs disappeared around 66 million years ago. So did many marsupial mammals, but placental mammals — the group that cats belong to — were also around in the Cretaceous and they replaced the marsupials in the next period, called the Paleogene. (Prothero)
This crisis occurred around the world and changed whole ecosystems (McGhee et al.), so it made sense to set a timeline boundary there, between the Cretaceous and the Paleogene, regardless of what the cause(s) of that mass extinction might have been.
And so it goes throughout our Phanerozoic.
Perhaps you have heard of other crises, like the end-Permian extinction.
Yes, the Permian was a period; so was the Triassic, which followed it and which also ended in a major mass extinction.
Life, extinction, and geological periods all fit together since Cambrian times. Unfortunately, geologists can’t be that precise with their Precambrian time scale yet.
There just aren’t that many fossils or clear-cut biomarkers from back then to work with.
Too, Precambrian rocks are usually so altered that it’s difficult for geoscientists to interpret them in ways that everyone can agree on.
Some researchers, Prokoph et al. in particular, note that large igneous provinces (LIPs) in the Phanerozoic appear to be connected with life crises and possibly could be used to divide up the Precambrian into more manageable periods of geologic time.
Here’s a video that shows what they are talking about, using the worst mass extinction of the Phanerozoic thus far as an example (part 2 gets a separate post:
Yay, fellow synapsid Lystrosaurus! (Dinosaurs were and are diapsids; this will make sense later in the cat-evolution series.)
Relax. These things happen on average every 30 million years, and the last one, in the Pacific Northwest, occurred roughly 17 million years ago.
We’ve still got some time before the next one. Probably.
I don’t know how widespread support is for the time-scale suggestion from Prokoph et al., but they do present a good argument for it.
Anyway, the debate about geologic time is for scientists. On a lay level, this LIP approach is a good way for those of us interested in cat evolution to experience some Precambrian excitement, as well as to better understand the setting in which lineages evolved that somehow led to the very first animals and plants.
In the cat evolution series, we’re currently working through cat-related evolutionary milestones associated with the Proterozoic’s surprisingly eventful “Boring Billion” years and the subsequent two major “snowball” phases.
At that point, according to some hypotheses, Earth may have been trying to return to a form of its original “stagnant-lid” tectonics with the supercontinent Columbia. (Mukherjee et al.; Palin and Santosh)
Unlike other rocky planets like Mars and Venus, Earth instead transitioned into the plate tectonics that still operates today.
This process involved LIPs and a few traces of those remain. Next time, let’s start with a look at those.
Featured image: Alex Kapov/Shutterstock
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