Evolution: lumpers vs. splitters

The NYTimes is running Dr. Olivia Judson's evolutionary biology BLOG again.

Her latest talks about the endless dialog in biology between lumpers and splitters--i.e., those whose view of evolution focuses on convergence vs. those who focus on historical happenstance. She talked about how cool it would be to run an experiment with 10,000 Earths for a few billion years to see what happened. Here's what I wrote in the Comments section:

I'd love to see that 10,000 Earths experiment. We'll need to add some conditions, though, based on the line of reasoning in the book Rare Earth, which I think has been pretty well substantiated.

1. The suns will all have to be yellow dwarfs like our sun (which is a common type, fortunately). Larger stars aren't stable enough--their temperature rises too quickly (in a cosmological timeframe) to allow any given planet to stay in that star's life zone (i.e. where a planet will have liquid water). Smaller stars build up heavier elements around the fusion core, then occasionally "burp," producing a solar blast that would nuke a planet in the life zone; plus that life zone would be so close to the star that a planet in that location would become gravitationally locked--one side hot enough to boil off the atmosphere, the other cold enough to freeze it.

2. The 10,000 Earths will need 10,000 Moons as big as ours. Our Moon stabilizes our Earth axially--that is, our North Pole is roughly perpendicular to the plane of the solar system in which the planets rotate around the Sun's equator.

Without the Moon the precession of the Earth around its poles would be much greater--it could even lean over so far that a pole faced the Sun. Life could exist on an Earth like this but probably nothing beyond bacteria, as its weather would be too violent for complex organisms.

3. To get our Moon, Earth got smacked by a protoplanet over 4B years ago, during the coalescence of the solar system; the Moon formed from crustal ejecta after this collision, first as a Saturn-like ring, then gradually forming the Moon (which is why it's all crust--no iron core).

None of this means there aren't 10,000 Earths with a moon like ours out there, rotating around a K2 star. It just means that unlike the densely populated universe science fiction generally promotes--planets with complex life like ours--Earths are few and faaar between.

That said, what'll happen on those 10K Earths--the ones that do have a humongous moon like ours & a nice stable K2 star?

Depends on whether you're a lumper or a splitter.

Splitters say it's like the Star Wars cafe--everything imaginable & then some.

Lumpers say maybe they'll mostly turn out like Earth.

Here’s my lumper pitch.

We started out thinking everything was fixed design, as per Genesis. Then we let go of that and assumed it was all chance. Now we know better. We’ve come to see how what we thought was accident turned out to be stark necessity.

For example, our eyes are close to our brains because nerve impulses travel too slowly to put ‘em any farther out. There are a bazillion more examples. What it boils down to is that just as our fantasy that life was everywhere, even on Venus and Mars, turned out to be wishful thinking, our fantasy that advanced life can be as diverse as we see in science fiction, that’s equally wishful thinking.

The proof for me came from scuba diving, which I’ve done all over the world, in depth (so to speak). You’d think the psychedelic variety we see on coral reefs would point me in the other direction, but after a while you start to see how much form follows function—and every critter’s character was hammered out on the anvil of convergent evolution.

And what ecological niche is more demanding than “advanced technology-using planetary top dog?” I bet this niche is mostly filled, across the universe, with bipedal, terrestrial hominids—and I could give you an evolutionary model that goes from coelocanthoids to us, showing how at every step every big alternative would have bit the dust.

People WANT the Star Wars café, and for them I’m just being a buzz kill. But the presumption that advanced life forms must be wildly divergent is just that: a presumption. Let me remind readers of the principle of mediocrity—what we see is average.This is a powerful tool for scientific understanding when we’re hypothesizing. The only technological advanced life form (TALF) we know of is us. Your default assumption should be that other TALFs would look pretty much like us unless you can produce a compelling evolutionary model for something else—not the obverse.