Over on the speculative evolution forum, Science Meets Fiction asked this cool question: “What might life look like if it had ten times as much time to develop since its world’s equivalent of the Cambrian Explosion?”
I can’t do it justice, but – what do you know – I tried anyway. Here goes:
This experiment seems to show that evolution doesn’t hit an asymptote and stop. Even in a completely unchanging environment, while there are diminishing marginal returns to optimization, they never diminish to zero. And then of course at some point the organisms themselves will start changing the environment.
1) That’s one way to go: on a very old world, all big environmental changes are biogenic. A mere asteroid impact doesn’t do nearly as much damage as the native life.
Evolutionary biologists slap you on the wrist if you talk about trends, but I’m going to anyway 🙂 One trend (noted by Stephen Jay Gould) is an increasing difference between the least and the most complex organisms. There is a minimum viable complexity and organism can have and still be called “alive,” but no maximum exists. Over time, simple chance will result in a longer and longer tail on the distribution graph in the direction of increased complexity. The same might be true for size, as well. A billion years ago, we only had microbes. Now we have microbes, blue whales, the Humongous Fungus, and Pando the aspen grove.
2) on a very old world, there are some very big organisms
A similar trend (or a result of the same trend) is increasing diversity and decreasing disparity. More and more species share a more and more recent common ancestor. In the Cretaceous, there were no plankton-eating aquatic mammals, no grazing mammals, no flying mammals, but there were multituberculates and triconodonts in addition to today’s placentals, monotremes, and marsupials. There were also gondwanatheres, docodonts, and morganucodonts, which lie outside of “mammalia” entirely! They all looked like small furry scampering things (low diversity) but they were less closely related than a squirrel is to a lion (high disparity).
3) Extend that into the future and every land animal is (for example) a kind of house mouse. Maybe EVERYTHING is a house mouse, from microbes (transposons and much-simplified transmissible tumor cells?) to forests (lots of endosymbiosis events).
Another trend is the creation of new niches. In the Cretaceous there were no grazers because there were no large grasslands. In the Cambrian, there were no large land plants at all, and therefor no ecosystems depending on them. It’s hard to imagine all that land area going to waste, but here in the Holocene we have low-productivity deserts, mountaintops, enormous volumes of ocean water, and even wilder, more barren places like the deep crust and the upper atmosphere.
4) More complex ecosystems in more places.
Thermodynamically, you can think of the Earth turning progressively more and more sunlight into waste heat. One way to think about deep future ecosystems is to imagine them becoming more and more efficient at collecting energy and sequestrating biomass.
5) Biogenic dyson-spheres? Kardishev III ecosystems?
And what about intelligence? I’ve been toying with the idea of an optimistic* deep future for intelligent life, where rather than destroying itself, it just keeps growing in complexity.
6) Everything on future Earth didn’t evolve from house-mice. Everything evolved from humans.
*optimistic if you like humans