This is the short, citation-free version of the How We Spread Across the Earth arc — the three-part story of how our species, born on a single continent, came to stand on all of them. It compresses those three entries — The African Dawn (#0016), Out of Africa (#0017), and Into the New World (#0018) — into one self-contained narrative.
This is a recent and, in the long view, abnormal condition. For almost the whole of the human story, the Earth held several kinds of human at once. As recently as fifty thousand years ago — a flicker, in the time-scales this project works in — our planet was home to Homo sapiens in Africa and the Near East, the Neanderthals across Europe and western Asia, the Denisovans somewhere in the vast Asian interior, the tiny Homo floresiensis on an Indonesian island, and another small human, Homo luzonensis, on a Philippine one.
We left the previous entry at a gate. Our species had existed for something like a quarter of a million years, almost all of it inside Africa. It had pressed against the northern exits more than once, during the wet intervals when the desert greened, and each time it had fallen back, leaving no lasting line beyond the continent. Then, sometime around seventy thousand years ago, the corridors opened again, and this time the people who passed through kept going. Everything in this entry follows from that crossing.
But before anyone could enter the empty hemisphere, they had to survive the cold that guarded its door. That is where the story begins.
The previous entry ended in the early Eocene, with the small, large-eyed, tree-dwelling primate Teilhardina moving rapidly across the Northern Hemisphere during a brief warm spike at the boundary between the Paleocene and Eocene epochs. The basic primate body plan — forward-facing eyes, grasping hands, social groups, long childhoods — was in place. What had not happened, and would not happen for another fifty million years, was that any branch of the primates climbed back down out of the trees.
For readers who want to dig in, the detailed entries — #0003 through #0010 — remain in the corpus, with the references, the disputed claims, and the working uncertainties intact.
This is the short story of how you came to be. Not how you, personally — though that comes into it at the end. How any human being came to exist, as the kind of animal we are, on the kind of planet we live on, in the kind of universe that contains us.
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.