Nature

Ghostly vestiges of our decaying skyscrapers jut out like skeletal remains casting long lonely shadows over the hollow planes where once thrived a sprawling Metropolis of life. Within this abandoned behemoths, a fading heartbeat of nuclear-powered luminescence persists — an enduring testament to our vanished civilization.

The question seems narrow, but the answer turns out to be most of what matters. By the time the period covered in this entry ends, the air had been remade, the ozone layer had formed, the oceans had begun to clear of dissolved iron, an entirely new kind of cell had been invented, sexual reproduction was in operation, and the first organisms had begun to live in cooperative bodies of many cells working as one. None of the species a casual modern observer would recognise existed yet. There were no animals, no land plants, no macroscopic fungi.

The previous entry ended in the late Carboniferous, in the swamp forests of what is now Nova Scotia, with a small lizard-like animal sheltering inside a hollow tree. That animal, Hylonomus, was an early amniote — a member of the lineage whose innovation, the amniotic egg, had freed vertebrate reproduction from the need for standing water. With that freedom, the dry interior of the continents was open to vertebrates for the first time.

How did the descendants of a small Carboniferous amniote come to include both an albatross and a human being?

In the previous entry I described what humanity at this time understands about how life began on Earth — the chemistry that produced the first replicating molecules, and the moment at which ordinary matter first acquired the property of making copies of itself.

In the previous entry I described the molecule of inheritance — how its four-letter sequence is copied across generations with very high but imperfect fidelity, and how the small errors that creep in during copying are the source of new hereditary variation. The previous entry closed with the observation that anything which helps a gene get copied into the next generation will, over time, become more common, and anything that gets in the way will, over time, fade.

The previous entry described the first two billion years of life on Earth — a long, almost entirely microscopic stretch in which the chemistry of cells was worked out, the atmosphere was remade, and the cooperative architecture of multicellular bodies was assembled in the oceans. By its end, eukaryotic cells were established and several lineages had begun to live in cooperative bodies of many cells working as one.

Where possible, I cite sources from which I made inferences so that future readers — should they recover the technical literature of this era — can trace specific claims back to their origins and assess them for themselves. References are listed, in numbered form, at the end of each entry.

How did living things arise on a planet that was, at first, not living?