The New York Times, May 27, 2015
For scientists who study human evolution, the last few months have been a whirlwind. Every couple of weeks, it seems, another team pulls back the curtain on newly discovered bones or stone tools, prompting researchers to rethink what we know about early human history.
On Wednesday, it happened again. Yohannes Haile-Selassie of the Cleveland Museum of Natural History and his colleagues reported finding a jaw in Ethiopia that belonged to an ancient human relative that lived sometime between 3.3 and 3.5 million years ago. They argue that the jaw belongs to an entirely new species, which they named Australopithecus deyiremeda.
While some experts agree, skeptics argue that the jaw belongs to a familiar hominid species, known as Australopithecus afarensis, that existed about 3.9 to 3 million years ago.
Studies like this one are adding fresh fuel to the debate over the pace of human evolution. Some researchers now believe the human family tree bore exuberant branches early on.
“I’m so excited about these discoveries I’m driving my friends crazy,” said Carol V. Ward, a paleoanthropologist at the University of Missouri. “It makes us stop and rethink everything.”
In the 1990s, the broad outlines of human evolution seemed fairly clear. Early human ancestors, known as hominids, evolved from an ancestor shared with chimpanzees about six or seven million years ago. These hominids were short, bipedal apes with small brains and arms and legs still adapted for climbing trees.
Until about three million years ago, experts thought, there weren’t a lot of hominid species. In fact, some researchers argued that most hominid fossils represented just a single species.
In 1974, the paleoanthropologist Donald Johanson and his colleagues found a fairly complete, 3.4-million-year-old skeleton in Ethiopia, which they nicknamed Lucy. The species was named Australopithecus afarensis, and many more examples have come to light, dating from about 3.9 to 3 million years ago.
Scientists had thought that hominid evolution became more complex just 2.4 million years ago. New species split apart from Australopithecus afarensis, at least a few of them coexisting in Africa.
One lineage, called Paranthropus, evolved powerful jaws it probably used to grind tough plant matter. Other hominids developed nimble hands, which they used to make stone tools for butchering meat. Eventually they evolved into tall, long-distance walkers.
These hominids belonged to the genus Homo, which produced our own species about 200,000 years ago.
But with new discoveries like Australopithecus deyiremeda, this eons-long story may need to change. Hominids may have become much more diverse much earlier than previously thought. Australopithecus afarensis may have had a lot of company.
In 1995, Ronald J. Clarke of the University of the Witwatersrand in Johannesburg and his colleagues discovered Australopithecus fossils in a South African cave. While the fossils have yet to be formally named, Dr. Clarke and his colleagues have started referring to the putative new species as Australopithecus prometheus.
Geologists initially estimated that the rock layer atop the bones was 2.2 million years old. But that research did not tell them exactly how much older the fossils might be.
More recently, Dr. Clarke and his colleagues have used new methods to date the rock layer in which the fossils were embedded. In April, they reported that Australopithecus prometheus was 3.67 million years old.
Yet another possible contemporary of Australopithecus afarensis lived in Kenya. In 2001, researchers reported the discovery of a flat-faced hominid skull dating back 3.5 million years. They called it Kenyanthropus platyops.
And even before Wednesday’s announcement, Dr. Haile-Selassie had been adding to the debate about early hominid evolution. In 2012, he and his colleagues reported finding 3.4-million-year-old foot bones in Ethiopia from a previously unknown hominid.
The long, grasping toes appear to have been better suited for tree climbing than those of Australopithecus afarensis, suggesting it belonged to a species of its own. Until scientists can describe more bones from its skeleton, it remains without a species name.
These early hominids may have been more mentally sophisticated than previously thought, scientists also have found. Until now, the oldest stone tools ever found dated back 2.6 million years — about 400,000 years after Australopithecus afarensis became extinct.
But last week, Sonia Harmand of Stony Brook University and her colleagues reported discovering tools in Kenya that they estimate to have been made 3.3 million years ago. The researchers suggested that the tools were made by Kenyanthropus, because its fossils come from rocks about the same age and in the same region of Kenya where the tools were found.
Dr. Ward, of the University of Missouri, said the evidence gathered so far pointed to a much earlier explosion of hominid diversity. “It changes our view of human evolution in a fundamental way,” she said.
Four or more species may have coexisted with Australopithecus afarensis. Some may have specialized in different ways of getting food, perhaps with newly developed stone tools, for example. Or they may have competed with one another.
The tools also hint that at least some of these early hominids were capable of more complex thinking than previously believed. “The stone tools represent a sophistication in how they use and manipulate objects,” Dr. Ward said.
Scientists have also shed new light on the transition from Australopithecus to Homo. In March, Kaye E. Reed of Arizona State University and her colleagues reported finding the oldest Homo fossil, dating back 2.8 million years. It has some anatomical features found only in Homo, such as narrow molars. But it has other traits, like a rounded chin, that make it look more like Australopithecus afarensis.
Dr. Ward said scientists now must trace Homo’s origins to one of the several hominid species that may have lived between three million and four million years ago — and figure out why the other species became extinct.
But some hominid experts remain unconvinced that the road to Homo took so many turns. Tim D. White, a paleoanthropologist at the University of California, Berkeley, argues that most of the new studies have been rushed into publication without careful peer review.
The 3.3-million-year date for the ancient stone tools, for example, “seemed quite sketchy to me,” Dr. White said. The tools could have been made hundreds of thousands of years later, he said.
Dr. White is also skeptical that the new fossils represent a wealth of new species. He suspects that most of them, including Australopithecus deyiremeda, are just Australopithecus afarensis.
“Lucy’s species just got a few more new fossils,” he said of Wednesday’s announcement.
The peculiar anatomical quirks described by other scientists are no more unusual than the variations found within living ape species, he said. When scientists discover a fossil, Dr. White warned, it can be easy to blow minor variations out of proportion.
“A piece of a mandible doesn’t tell you much,” he said. “Whenever you have small samples, you run a very real risk of mischaracterization.”
Dr. White said it would be wiser to assume that new fossils belonged to documented species, like Australopithecus afarensis, instead of hypothesizing a new species with every new fossil. As he sees it, human evolution isn’t the bushy tree that Dr. Ward describes.
“A saguaro cactus would be the metaphor,” said Dr. White.
Even Dr. Ward expects that scientists will eventually decide some of the new “species” really aren’t species. Even so, she predicted that early hominids would remain more diverse than traditionally thought.
“There were at a bare minimum two hominids around at that time, and perhaps three or more, which is exciting and important however it falls out,” she said.
Copyright 2015 The New York Times Company. Reprinted with permission.