Map of life expectancy at birth from Global Education Project.

Sunday, June 04, 2017

The Greatest Story Ever Told

The Energy Expansions of Evolution, by Olivia P. Judson, in Nature Ecology.

Do read it -- you'll enjoy it and you'll learn something. Then read my commentary.

This is the story, in part, of how Gaia got her cloak of green; and subsequent major developments. It also helps us to think about the so-called Fermi paradox -- why don't we see any evidence of extraterrestrial technological civilizations, when there's no particular reason to think we're unique or special? Finally, it's just a story of awesome grandeur that should make us focus very, very hard on overcoming the present crisis facing humanity. We are incredibly lucky to find ourselves on such an unlikely planet, let's not kick it away.

Life is believed to have appeared on earth some time before 3.7 billion years ago. But for a long time, it didn't amount to much. Extraterrestrial visitors to our we rock likely wouldn't even have noticed the microscopic  self-replicating polymers encased in oily bubbles, that were probably found only near deep-sea geothermal vents and possibly some other isolated locations. Our old idea of life originating in shallow, sun-lit tidal pools is probably wrong, because the earliest life couldn't exploit sunlight as an energy source. The sun warmed the rock and water, which radiated the heat back into space. In between, it didn't do anything.

About 3.7 billion years ago, organisms emerged that could exploit the energy of sunlight. But they didn't split CO2 and water to make hydrocarbons and emit oxygen. The early forms of photosynthesis were less efficient and required access to existing organic carbon. So one-celled organisms became more numerous, but no major transformation resulted. (They probably emitted methane, which helped warm the planet.)

Then, about 2.4 billion years ago, oxygen began to build up in the earth's atmosphere. This was because a group of organisms called cyanobacteria had evolved oxygenic photosynthesis. Actually they had  evolved a few hundred million years earlier; it took a while for the oxygen build up to get going. But cyanobacteria could now inhabit a far wider array of niches and create far greater biomass. A consequence was that it was now possible for other organisms to make a living just by eating others, but another unlikely event had to occur before that became a major industry.

Then an unlikely event occurred. An organism called an archaeon acquired a bacterium as an endosymbiont. The bacterium was very efficient at converting oxygen and nutrients into ATP, the cellular fuel. The resulting organism, called a eukaryotic cell, could grow big and complex, and they started making a living by engulfing and digesting smaller cells. Then, one of them acquired as a second endosymbiont a cyanobacterium, and the plant cell was born.

Because of the way they reproduce, eukaryotes could form complex multicellular organisms. We started to see big things that could move around and seek food. They even started eating each other. This happened around 575 million years ago. As Judson tells us:

[W]ith animals would soon come a powerful new force of nature: the acquisition of energy through the active hunting and eating of other life forms, especially, other animals. This would produce a radical shift that, within a mere 40 million years, transformed the Earth. Before this epoch, ecosystems were microbial. The advent of widespread flesh-eating launched the Phanerozoic, triggering an enormous increase in organism size85, a new tempo of macroevolutionary change86,87, new kinds of ecosystems86,​87,​88, and an increased impact of life on the fabric of the planet87.
The collective term for these critters is metazoa. A million years ago or so, one of the metazoans started to deliberately set fires in order to cook its food and perhaps ward off predators. Then it started to use fire to extract metals from ores, and shape them into highly effective tools. Then it discovered abundant fuel in the ground that could be used to power machinery -- you know the rest.

If you really think about it, this chain of events seems quite unlikely. Maybe it isn't really and it would happen sooner or later on any properly situated planet. Who really knows? But we shouldn't be surprised if it is very rare in the universe. We really need to start appreciating it.


Daniel said...

Great link and well said.

Some argue that the L factor in the Drake equation answers the Fermi paradox.

Cervantes said...

Well, we may be on the brink of doing ourselves in but it doesn't have to be that way. (L is the lifespan of technological civilizations.)