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The Crab Nebula -- 6,000 light-years away -- is the remnant of a supernova explosion. It was observed by man almost 1,000 years ago, in the year 1054. (ESO)

From The Big Bang To Intelligent Life: The Four Eras Of Astrobiology

Dec 14, 2011

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This past Sunday I was asked to give an opening lecture at an international conference on astrobiology, linking cosmology to the emergence of life on Earth and other planetary platforms. We, and any other alien creature that might exist, are products of the same laws of physics and chemistry that operate across the cosmos.

At its most basic level, life is a set of self-sustaining chemical reactions capable of metabolizing energy and, crucially, of reproducing according to Darwinian evolution.

So, living creatures are self-organizing bundles of molecules capable of creating copies of themselves. Since molecules are made of atoms and atoms are made of protons, neutrons and electrons, living creatures need, as their most basic ingredients, the particles of matter that fill the cosmos.

The first age of astrobiology can thus be called the physical era, from right after the big bang to the formation of the first stars at a few hundred million years after the bang. This is when material particles first emerge and organize and into light nuclei (namely, hydrogen, helium, and lithium) at a few minutes after the bang. Then, at about 400,000 years after the bang, protons and electrons combine to form hydrogen atoms. Once the universe had hydrogen coalesced into large clouds, gravity did the rest. Aided by dark matter — a kind of matter of yet unknown composition but gravitationally active — these clouds of hydrogen grew and became unstable, collapsing into the first stars.

These stellar behemoths lived short lives, exploding with tremendous fury in gigantic supernova events, generating the chemicals familiar from the periodic table, the raw ingredients of life, and spreading them across space. This marks the beginning of the chemical era: the universe now has chemistry and larger atoms that can accumulate into proto-stellar disks.

The first galaxies are formed within the first billion years after the bang; the stellar population grows and diversifies. At a few billion years after the bang, the universe has countless galaxies, stars, and planetary systems. The biological era can now start in earnest.

Certainly, at about four billion years ago, life emerged on Earth out of the raw ingredients formed from dying stars in our neighborhood. Quite possibly, it also emerged in other spots across the universe, before and after it emerged here.

The fourth era, which I call the cognitive era, is quite recent, at least as far as we know, starting here under one million years ago. Of course, it could have started elsewhere a billion or two billion years before it did here, but not much earlier than that. Life takes a long time to evolve from unicellular to multicellular to intelligent, if it does at all. The kinds of life a planet bears is contingent on its particular history. The life we find here exists only here. More crucially, if life is ubiquitous, intelligent life might not be.


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