Pathogenesis

Pathogenesis

Chapter 1

Paleolithic Plagues

History makes no sense without prehistory, and prehistory makes no sense without biology.

—E. O. Wilson

Rediscovering Middle-earth

The idea of a world inhabited by multiple human and humanoid species will be familiar to readers of fantasy literature. Take, for example, the Fellowship that accompanies Frodo Baggins on his journey to dispose of the One Ring in the fires of Mount Doom. Aragorn and Boromir are Men, a term used to denote both male and female humans. Frodo, Sam, Merry and Pippin are Hobbits, closely related to Men but roughly half as tall and with oversized, furry feet. Then there is Legolas, a slender and pointy-eared Elf with a superhuman sense of sight and hearing. And Gimli is a Dwarf, belonging to the short, thick-set warrior-like people who live in the mountains of Middle-earth.

J. R. R. Tolkien did not create this legendarium from scratch. His fantasy world was strongly influenced by the Germanic mythology that he studied in his day job, as a professor of Anglo-Saxon at Oxford University. This is why Tolkien claimed to have discovered rather than invented Middle-earth. Over the last two decades, researchers have uncovered an array of evidence that has transformed our knowledge of the world that early humans inhabited. New archeological discoveries, combined with advances in the technology used to analyze the DNA retrieved from ancient skeletons, clearly demonstrate that for most of the time Homo sapiens has been around—from about 300,000 to 50,000 years ago—the planet resembled Tolkien’s Middle-earth or a Norse saga more than the world we occupy today. Although our ancestors didn’t live alongside Hobbits, Elves and Dwarves, they shared the earth with a rich cast of human species.

Geneticists estimate that our last common ancestor with chimpanzees dates to between 6 and 8 million years ago. Just over 3 million years ago, proto-humans were habitually walking on two legs but the size of their brains and bodies had hardly changed—as demonstrated by “Lucy,” the female skeleton discovered in Ethiopia in 1974 by archeologists as they listened to the Beatles’ song “Lucy In The Sky With Diamonds.” Homo erectus, or “upright man,” appears in the fossil record about 2 million years ago. With relatively long legs, short arms and a large head, Homo erectus is the earliest example of a species that looks recognizably human. They were the first species of humans to migrate out of Africa, and within a relatively short period of time they managed to spread across much of the Old World. Their remains have been found near the southern tip of Africa, in the Caucuses, northern China and Java.

Our own species evolved from Homo erectus. The first known skeletal remains with the modern anatomical features typical of Homo sapiens are the fossilized bones of five people who died some 100 kilometers from Marrakesh about 315,000 years ago. For most of the time since then, they remained more or less exclusively in Africa—although our ancestors’ remains have been found everywhere from Morocco to the Cape. Homo sapiens wasn’t the only species of humans living in the continent, however. There is both archeological and genetic evidence that we coexisted in Africa with a variety of other species of humans.

Neanderthals also evolved from Homo erectus. They diverged from our own species between three-quarters and half a million years ago, when a group of archaic humans migrated out of Africa and ended up in Europe. Homo neanderthalensis retained so-called archaic features—that is, lower braincases, heavier brows and less prominent chins—all of which distinguish them from us anatomically modern humans. Neanderthals were also taller, heavier, stronger and had slightly bigger brains than Homo sapiens. Their fair skin helped them absorb sunlight—which is crucial for making vitamin D—and their large, frequently blue eyes enabled them to see in the dark European winter. Neanderthals eventually spread out over much of Western Eurasia; their remains have been found from Gibraltar in the west to the Altai Mountains in Siberia in the east.

Over the last two decades, scientists have discovered several more species of humans that were alive at the same time as Homo sapiens. Denisovans split from Neanderthals not long after they’d ventured out of Africa and went on to occupy the eastern part of Eurasia. The only physical traces of this species are a few bone fragments uncovered in caves in the Altai Mountains and on the Tibetan plateau. Anatomically, Denisovans would have looked similar to Neanderthals although they appear to have had much bigger teeth, and they carried a number of gene mutations, including one that affected red blood cells and allowed them to live comfortably at high altitudes. Homo floresiensis lived on the Indonesian island of Flores. They are colloquially referred to as Hobbits on account of their height—they stood just over a meter tall—and disproportionately long feet. One theory suggests that Homo floresiensis is descended from Homo erectus, who arrived there about a million years ago and then became isolated by deep waters. Homo luzonensis is another extinct, small-bodied human species that was discovered in 2019 on the island of Luzon, in the Philippines. Their curved fingers and toe bones suggest that they retained the climbing abilities of our pre-human ancestors.

So for the first quarter of a million years, Homo sapiens lived in Africa alongside other species of humans, and yet more species of human inhabited Europe and Asia. Then, between 50,000 and 40,000 years ago, something astonishing happened. Within a few thousand years, Homo sapiens burst out of Africa and quickly spread across the world—from western Europe all the way to Australia. At the same time, all other species of human vanished from the face of the earth. The most recent trace of Homo luzonensis and Homo floresiensis is from 50,000 years ago. The last evidence of Denisovans dates to between 49,000 and 43,000 years ago, although they may have held out in isolated parts of New Guinea for longer. Neanderthals appear to have survived until between 41,000 and 39,000 years ago. The expansion of Homo sapiens and disappearance of other species fundamentally transformed the planet and laid the foundations for the world we inhabit today. Why this happened is one of the biggest mysteries of human prehistory.