The changes we’re starting to go through now are so big that we need to step back and take a very long view to get any sort of handle on them. I’m struggling to do this now. This book has been very helpful:
• Tim Lenton and Andrew Watson, Revolutions That Made the Earth, Oxford U. Press, Oxford, 2011.
There’s a lot in it, and I’d love to tell you about all of it… but for now, let me just list 8 major transitions life on Earth may have gone through:
1. Compartments. This transition takes us from self-replicating molecules to self-replicating molecules in ‘compartments’—membranes of some sort. A compartment separates ‘self’ from ‘other’, and it’s crucial to life as we know it. When did this transition happen? Certainly after 4.4 billion years ago, when the Earth first got a solid crust. Probably after 3.85 billion years ago, when the Late Heavy Bombardment ended. Certainly before 3.3 billion years ago, when the earliest well-established microfossils are found. Probably before 3.8 billion years ago, which is the age of the Isua greenstone belt—a formation that contains graphite specks with less carbon-13 than average, a hint of life.
2. Groups of genes. This transition takes us from independent self-replicating molecules to self-replicating molecules linked into long chains, probably RNA. When did this happen? I have no idea; I’m not sure anyone does. Probably sometime between 4.4 and 3.3 billion years ago!
3. Genetic code. This transition takes us from a world where RNA both stored information and catalyzed reactions to a world where DNA stores the information used to build for proteins, which catalyze reactions. When did this happen? Again, probably sometime between 4.4 and 3.3 billion years ago!
Eukaryotes—like animals, plants, fungi and protists—have cells with lots of internal parts called organelles. Here are some things you might see in a eukaryotic cell:
It’s now believed some organelles were originally independent prokaryotes that got swallowed up but survived as symbiotic partners: so-called endosymbionts. The evidence is especially good for mitochondria and chloroplasts, which have their own DNA. When did this transition occur? Some experts say around 1.85 billion years ago. Nick Butterfield has seen fossils of red algae dating back to 1.2 billion years ago, so eukaryotes were definitely around by then. The authors of this book date eukaryotes to “roughly 2 billion years ago, give or take 0.5 billion years.”
5. Sex. This transition takes us from populations of asexually reproducing clones to populations that reproduce sexually. When did this happen? Roughly around the time eukaryotes arose. The authors write:
We would like to know if the evolution of sex is really separate from the evolution of eukaryotes, or whether the two are so closely related that sex co-evolved with the eukaryotic cell. It would help if we knew precisely why organisms bother with sex, but we don’t.
6. Cell differentiation. This transition takes us from single-celled protists to multi-celled animals, plants and fungi where different cells specialize to play different roles. When did this happen? The oldest known animal fossils are some early sponges in the Trezona Formation in South Australia… they go back 665 million years. Plants may go back 1.2 billion years, and fungi perhaps around 1.4 billion years. Just for fun, here’s a typical plant cell:
but of course the point is that thanks to differentiation, different cells in the organism look different!
7. Social colonies. This transition takes us from solitary individuals to social organizations such as colonies of ants, bees and termites, or the somewhat different societies of birds and mammals. Sociality has arisen independently many times, but it’s hard to say when because it’s hard to find fossil evidence! In the early Triassic, about 250 million years ago, we find fossilized burrows containing up to twenty cynodonts of a type known as Trirachodon:
Cynodonts are classified as synapsids, a group of animals that includes mammals but also ‘proto-mammals’ like these. By the late Triassic, there’s also evidence for social behavior among termites. It would be funny if proto-mammals beat the insects to sociality. I bet the insects got there first: the fossil record is not always complete!
8. Language. This is the transition from societies without language (for example, earlier primates) to societies with (for example, us). When did this happen? Alas, it’s even harder to read off the beginning of language from the fossil record than the arrival of social behavior! I’ll just quote Wikipedia:
Some scholars assume the development of primitive language-like systems (proto-language) as early as Homo habilis, while others place the development of primitive symbolic communication only with Homo erectus (1.8 million years ago) or Homo heidelbergensis (0.6 million years ago) and the development of language proper with Homo sapiens sapiens less than 100,000 years ago.
So that’s a list of 8 ‘major transitions’! With each one we get a higher level of organization, while preserving the structures that came before. At least that’s true of the first 7: the last is a work in progress.
In fact, this list was first propounded here:
• Eörs Szathmáry and John Maynard Smith, The major evolutionary transitions, Nature 374 (1995), 227-232.
and these authors expanded on their ideas here:
• Eörs Szathmáry and John Maynard Smith, The Major Transitions in Evolution, Oxford U. Press, Oxford, 1995.
I haven’t read that book yet, alas. Lenton and Watson actually argue for a different, shorter list:
1. The origin of life, before 3.8 billion years ago.
2. The Great Oxidation, when photosynthesis put oxygen into the atmosphere between 3.4 and 2.5 billion years ago.
3. The rise of complex life (eukaryotes), roughly 2 billion years ago.
4. The rise of humanity, roughly 0 billion years ago.
They consider these to be the “truly difficult events that may have determined the pace of evolution”.
Of course the latest revolution, humanity, is not complete. There is no guarantee that it will have a happy ending. Lenton and Watson sketch several alternative futures:
I’ll say more about these later. This is what Azimuth is ultimately all about.