Late Sunday night I had just finished revising chapter 16, Human Origins, in my genetics textbook when the news releases for the upcoming issue of Science came in and turned much of what I’d just written upside down.
Homo, traditionally described as a cluster of rather narrowly-defined branches on our family tree — distinct species — might be just one sort of animal. Should I delete H. habilis, H. erectus, and the other more ancient H.’s, before plunging into the various archaic humans (H.’s idaltu, neanderthalensus, and denisova)?
I’ve often wondered how paleontologists distinguish species of australopithecines, who were ancestral to and contemporary with Homo, based on a few fossils – like the famed Lucy and Dikika from Ethiopia, separated in time by 300,000 years, and the trio of footprints, presumably parents and a child, left millions of years ago in Tanzania. Isn’t paleontology a little like declaring that my backyard is home to different species of rodent for each diverse collection of Tamius striatus that my cats drag in each day? Last week they brought me one with a huge head – was it a macrocephalic chipmunk, or a different species?
SKULL 5 REWRITES THE TEXTBOOKS
I almost never change anything in a textbook based on a single report, but skull 5, and the four others near it, reported in Science this week may be an exception. The journal nicely put together a phone press conference so three of the researchers could describe what they’d found.
I’ve learned from past hominin discoveries (hominid and hominin are not interchangeable, but I’ll leave that to the textbook) that describing a skull that makes headlines actually represents many years of work. The jaw that fits skull 5 was unearthed in 2000, five years before its cranium, in Dmanisi, Georgia, a location that one of the researchers, Marcia Ponce de León, from the Anthropological Institute at the University of Zürich, called “a wonderful place that every paleoanthropologist dreams of.”
And what a specimen! “The preservation is exceptional. Many previously unknown aspects of the skeleton can be studied, and in more than one individual,” said David Lordkipanidze, director of the Georgian National Museum. The site also held remains of plants and other animals that lived in the forests or on the steppes of the temperate and humid area. Stone tools found near the skulls and animal bones bearing cutmarks indicate that these forebears knew how to prepare meat. The fossils provide “a real snapshot at one point in time of an ecosystem from 1.8 million years ago,” Lordkipanidze added.
The discovery that will lead to rewriting the textbooks stems from the finding of a mini paleopopulation – fossils from more than one individual – providing an unprecedented peek at physical variation. The first task was to show that the fossils were indeed from the same time and place, like girls at a Justin Bieber show. Next came comparisons of the skulls to each other, using classical as well as newer computer-aided methods to evaluate and quantify three-dimensional features. Finally, the anthropologists stepped back and eyeballed the variability.
“The amount of variation within the 5 individuals is equal to the amount we see in any 5 human individuals picked out at random, or 5 randomly picked chimps or bonobos,” said Christoph Zollikofer, a neurobiologist at the University of Zurich’s Anthropological Institute. That is, if the five Homo at the site in Georgia were considered separate species, so might the members of the Rolling Stones.
That variability implies that rather than several Homo species, a single lineage with diverse features emerged. “We’re pretty sure that the variation we see in Dmanisi is within a paleospecies. We infer that the variation in the fossil sample is no more than variation in Homo erectus,” the best-studied type, said Zollikofer.
But the ancestor who left behind Skull 5 didn’t look much like traditional views of early Homo, said Ponce de León. “The combination of features is puzzling. The braincase is very small, around a third of that of a modern human at 546 cc – that was unexpected. But the face is quite large, with massive jaws and teeth that are big and large.”
DNA EVIDENCE FROM ARCHAIC HUMANS ALSO BLURS SPECIES LINES
Just as Skull 5 has spurred a rethinking of species boundaries back near our beginnings, genome sequencing has blurred lines more recently by revealing that modern Europeans have about 2 percent Neanderthal DNA, and various groups – Australian aborigines, Polynesians, Melanesians, and some others – have about 3.5% Denisovan DNA (a recent ancestor known from the DNA in a finger and two molars). I’d prefer to use the language that Bill Maher would use to describe the goings on that must have led to archaic DNA infusions into our genomes, but scientists politely call it “admixture.” The narrative that seems to be emerging is that whenever ancient human populations expanded or migrated enough to meet, sex happened.
The topics of “out of Africa” and “admixture” are where I appreciate my own antiquity, for I clearly remember the bickering that went on in the late 1980s and throughout the 1990s about whether modern humans originated solely in Africa and replaced later migrants (“the single origin” hypothesis), or that waves of early peoples left Africa and persisted in small populations outside Africa, an idea known as the “multiregional” or “regional continuity” hypothesis. Coverage went back and forth in my textbooks for several editions.
The single origin hypothesis grew out of the discovery of “mitochondrial Eve” in 1986. This was initially the work of post-doc Wesley Brown at the University of California, Berkeley, who used the revved up mutation rate of mitochondrial DNA (mtDNA) as a molecular clock. Brown compared mtDNA from 21 people of diverse ethnic background, and soon after, because Brown didn’t have sufficient information on the geographic origins of the 21, the late Allan Wilson (the father of molecular clocks), grad student Rebecca Cann, and Mark Stoneking expanded the mtDNA sample to 147. They tallied the sequence differences, applied the mutation rate, and traced back the last common ancestor of us all to a female from about 200,000 years ago, whom the media immediately named mitochondrial Eve. (Mitochondria are inherited only from mothers because the sperm’s unfortunate mitochondria are left outside the egg after the head burrows in and the tail is discarded.)
The trunk of the deduced evolutionary tree that bore mitochondrial Eve was firmly rooted in Africa. We are all her children. But Africans have the most varied genomes, and given rates of mutation, that means their ancestors go back the farthest. Jane Gitschier’s 2010 interview with Rebecca Cann in PLOS Genetics discusses the hate mail and threats she got following the media frenzy over the metaphorical Eve.
(The debate resurfaced in 2011 when Michael Hammer from the University of Arizona and his team discovered DNA sequences from archaic humans in isolated groups in sub-Saharan Africa. This finding added the twist of yet another archaic group, but one that stayed in Africa. And I haven’t even touched on the resetting of evolutionary clocks fueled by discovery of de novo mutations from sequencing genomes of parents and their children – data that are sometimes at odds with results from direct analysis of ancient DNA from preserved bones.)
I duly covered both views of the origin of modern humanity – single origin vs multiregional — in the six incarnations of my intro bio textbook “Life.” I leaned towards the “single origin” Eve camp, which had more of a consensus, although always aware that some fossil evidence pointed to an Asian origin. Then in the later ‘90s things got too nasty for a textbook, so I shortened the coverage, but wrote about the controversy in a cover story for The Scientist in 2004. I seem to recall a scientific meeting where attendees nearly came to blows over the dueling hypotheses.
Throughout the debate, people did try to bridge the gap, to explain all the data. I wrote in the second edition of Life, in 1995, “Possibly, some combination of the single origin and regional continuity models is closest to the truth. Perhaps African peoples migrated to Asia and Europe, where they met and mixed with other early humans, forming the gene pools we ultimately arose from.”
The story of our origins, whatever it may be, beautifully illustrates how we do science, how we think up and then set about disproving hypotheses. The phrase “scientific proof” always makes me cringe, for science doesn’t work that way. New results force us to re-evaluate what we thought we knew, and we can’t predict what technologies not yet invented will enable us to see. To say we’ve proven anything about the natural world is arrogance. Instead, science is built on evidence and interpretation. Both change, evolve.
The scattered skulls of fossil evidence for many years suggested a scenario of different species of Homo. Now discovery of Skull 5 counters that view, that instead, Homo was one type of animal. Similarly, mitochondrial Eve told us one tale, and the genome sequences of Neanderthals, Denisovans, and ourselves are now telling us another. The evidence doesn’t really contradict or even disprove – it provides glimpses that talented anthropologists and geneticists are assembling into an increasingly detailed portrait of our past.