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Who Else Is In Our Species?

If you're reading this--and if you have neurons for brains rather than microprocessors--I'm going to assume you're a member of my species, Homo sapiens. No other living species has ever shown a capacity for reading email newsletters.

Of course, that was not exactly what Carl Linnaeus had in mind when he named our species in the eighteenth century. But he did have language in mind when he chose sapiens as part of our name--Latin for wisdom or sagacity. We may have produced language with our throats or hands thousands of years ago, but in Linneaus's day it also traveled on printed pages. Now it flows across the Internet.

A century after Linnaeus established our species, Darwin developed a theory in which our species arose, like all other species, through evolution. Since then, scientists have determined that we share a common ancestor with chimpanzees and bonobos that lived roughly seven million years ago. Clues to that seven million years come from fossils as well as ancient DNA preserved in bones and soil.

In that seven million years, evolution continued to branch like a tree, as new lineages of hominins evolved. Paleoanthropologists have recognized some of those lineages as species. They have names like Australopithecus afarensis and Homo habilis. Paleoanthropologists have also argued about those names, trying to lump some species together or split species in two. But as messy as those debates can be, they get even messier as we close in on the present day.

In the past few hundred thousand years, the fossil record documents hominins that looked a lot like living humans--close enough that many paleoanthropologists are comfortable calling them Homo sapiens. There are some other fossils that are so different that they clearly belong to another species. Homo floresiensis, a small-brain hominin that lived on the Indonesian island of Flores, is a good example. But then there are others that leave scientists scratching their heads. Homo sapiens or something else? In or out?

Consider Neanderthals. Their fossils span a period from roughly 200,000 years ago to 40,000 years. They sported an anatomy that was a bit different from the bodies of living people. They didn't have a chin like ours, for one thing. But when artists make reconstructions of Neanderthals, like the one I posted above, they don't look all that different from people today.

Fred Smith, a paleoanthropologist at Illinois State University, has been considering Neanderthals for a long time. Comparing their fossils to the bones of living people, he decided in the 1980s that Neanderthals belong to Homo sapiens. He would classify them as a subspecies, Homo sapiens neanderthalensis.

At the time, bones were pretty much all Smith had to go on to make his judgment. But in later years, scientists figured out how to pull Neanderthal DNA out of their fossils and compare it to ours. It indicates that we share a common ancestor with Neanderthals roughly 600,000 years ago. They branched off, but then modern humans and Neanderthals interbred several times, starting perhaps as far back as 270,000 years ago. The new evidence, Dr. Smith wrote last year, has only strengthened his conclusion. We are all one species.

But when the Italian biologists Andra Meneganzin and Massimo Bernardi looked at the same evidence, they came to a different conclusion. By several different measures, we are distinct from Neanderthals--so distinct that they deserve a species of their own: Homo neanderthalensis.

I contacted Smith not long ago to ask him about Menganzin and Barnardi's paper. I wondered if they had changed his mind. They hadn't, Smith told me. He still sees us as one species.

But Smith said that he can understand how Menganzin and Barnardi see things differently. “I can’t prove they were subspecies, and not species,” he said. “They have a defendable argument.”

The trouble does not lie in ourselves, however. There are similar debates raging about many species of animals, plants, fungi, and bacteria.

“The whole question of what a species is biologically—it’s a murky problem,” Smith said.

I am drawn to murky problems. Life itself is a murky problem. As I write in my book Life's Edge, scientists don't agree on what life is, and they've put forward hundreds of definitions for it.

Species are a murky problem too, as I wrote recently in The New York Times. For my feature "What Is A Species, Anyway?" (gift link) I write about the disagreements biologists have about which concept of a species should drive their research.

For biologists who study living animals and plants, there is an urgency to this debate, because they are trying to map the world's biological diversity as much of it is veering towards extinction. For good or for bad, species remain the universal currency by which we measure that diversity--and by which we make decisions about what to preserve.

I also recently wrote a piece for the Times that tackles this murky problem closer to home, on our own branch of the evolutionary tree. Smith and other scientists have been mulling whether Neanderthals are another species or not for generations. But over the past decade, scientists have discovered a new group of humans that raise those same questions all over again: the Denisovans. As I write in "On the Trail of the Denisovans" (gift link), Denisovans make those questions even more murky and interesting than before.

Denisovans were a distinct branch of the human evolutionary tree. They split off from Neanderthals about 400,000 years ago. So the first question that Denisovans raise about species is whether they form a species of their own, or belong to the same species as Neanderthals.

We know that Neanderthals and Denisovans interbred. In fact, scientists have amazingly found a fossil of a female human with half Neanderthal DNA and half Denisovan DNA. And yet, despite their ability to interbreed, the DNA of Neanderthals and Denisovans stayed recognizably distinct for hundreds of thousands of years. What's more, it appears that they occupied different ranges--Neanderthals to the west, Denisovans to the east. The hybrid Denisovan, found in Siberia, might have lived in a narrow contact zone between the two groups.

So: does all that justify calling them Homo denisova?

Denisovans are just as closely related to modern humans as Neanderthals, sharing our 600,000-year common ancestor. And, as I write in the Times, scientists are finding lots of evidence in living people of interbreeding between modern humans and Denisovans. In places like New Guinea and the Philippines, Denisovans may have interbred numerous times with modern humans who arrived there, perhaps as recently as 25,000 years ago. So perhaps Denisovans belong to our own species.

Whether they could read this newsletter I will leave for future generations to decide.

Debating the Biggest Whale, and more...

Since my last email, I wrote about some other interesting developments in biology (all gift links):

Woolly mammoth organoids? Scientists have created the first elephant stem cells in a dish, which opens the way to lots of interesting new science.

A debate over the heaviest animal in history: Scientists dispute a recent claim that a whale that lived 39 million years ago holds the record. They say blue whales are the heaviest animal ever--and that they weigh a lot more than people thought.

Discovering Downs syndrome in the fossil record: ancient bones have extra chromosomes, it turns out.

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