Some old bones made big headlines this week, as paleoanthropologists reported that they had identified the oldest Homo sapiens fossils yet.
Freshly dated to be about 300,000 years old, a collection of fossils and stone artifacts unearthed in Morocco could push the origin of our species back 100,000 years, scientists say in two papers published Wednesday in the journal Nature.
Such a dramatic new date in human prehistory may seem shocking, especially to people who expect evolution to follow a tidy linear progression. But it's actually indicative of how this science works – and how understanding of evolution itself has shifted.
Paleoanthropology is kind of like a connect-the-dot puzzle, says Shara Bailey, a coauthor of one of the Nature papers and a paleoanthropologist at New York University. “But you don't have any numbers on it and only a quarter of the dots are there. Will you be able to recreate what that puzzle is? Not necessarily. But as you get more dots and you can put numbers on it, you can get a clearer picture of what that puzzle is actually supposed to be.”
Each new fossil and date is a boon for scientists, she explains, because it adds another dot and number to that sparse picture.
One reason that picture of early humanity is so sparse is that scientists didn't always know where to look for early human fossils, or have the funding or permission to search willy-nilly. Early in the process of piecing together the story of human prehistory, many human-like fossils were found in Europe, so some supposed that Homo sapiens first arose on that continent. But, as Professor Bailey points out, that's just where dense human activity might accidentally uncover fossils, and where a lot of scientists were looking.
Over the decades, as much older fossils have been found in Africa and genetics research has also pointed to an African origin of modern humans, that narrative shifted.
Before this North African site claimed the title of oldest Homo sapiens site, a 195,000-year-old site thousands of miles away in Ethiopia wore that crown. And with other similarly aged Homo sapiens sites nearby, some scientists pointed to East Africa as the birthplace of our species. Now researchers say the Moroccan site, called Jebel Irhoud, adds a new wrinkle.
“We're sort of incrementally inching in upon a better description of nature,” says Ian Tattersall, a paleoanthropologist and curator emeritus with the American Museum of Natural History, who was not involved in the new research.
He says “everything has changed completely” since he first stepped into the paleoanthropology field half a century ago, “and it would be very hubristic of me to suppose that things won't have changed just as much in another 50 years.”
An evolution of ideas sheds new light
The Jebel Irhoud fossils themselves are an example of how paleoanthropology has evolved over the decades.
The prehistoric site was an accidental find in the early 1960s. During mining operations, a section of earth collapsed, revealing various bones – including a nearly complete hominin skull. At the time, six hominin fossils were collected from the site.
Those six fossils left paleoanthropologists scratching their heads. The site was thought to be around 40,000 years old, but the fossils displayed a combination of primitive and modern features that didn’t fit the current thinking about evolutionary timelines.
But with fresh excavations of the site starting in 2004, pieces of the puzzle began to fall into place.
In the decades since the first fossils emerged from the mine, evolutionary models have continued to shift, especially as scientists have developed more advanced dating techniques. These revised dates are older, and make the Jebel Irhoud hominins seem less like oddballs, as they now seem to have a place near the base of the Homo sapiens clade.
Dating has been “a stumbling point for a lot of people,” says Christian Tryon, a paleolithic archaeologist at Harvard University who was not involved in the new research. It's challenging to pin down an exact age of an archaeological site and they often come with uncertainties and wide margins of error, he says.
In this case, two newer dating techniques place the Jebel Irhoud site somewhere between 254,000 and 349,000 years old.
Messy evolution makes for messy paleoanthropology
Another thing that often muddies the waters of paleoanthropology is the old model of evolution as a linear “slog from primitiveness to perfection,” Dr. Tattersall says.
In that view, modern humans evolved out of a long chain of hominins, starting with the primitive primates and becoming increasingly sophisticated and more like us over time. The Jebel Irhoud fossils would probably be considered our direct ancestors, if that was how evolution worked.
But evolution is not linear and instead has many branches. And as such, the fossil record shows that human prehistory was a story of “tremendous evolutionary experimentation with different takes on what a member of the genus Homo could be,” Tattersall says. “And this is a wonderful illustration of that.”
In fact, Dr. Tryon says, “the way evolution works, extinction is the norm.”
With that mindset, the new fossils could be members of a population of early Homo sapiens relatives that ultimately died out. Or perhaps, as authors of the new papers suggest, populations across Africa may have been evolving different characteristics and then interbred later to mix those features to yield what we now see in human skeletons.
Sorting out those evolutionary patterns may be tricky. Africa is a large continent, and the collection of Homo sapiens specimens is “woefully small,” says Bernard Wood, a paleoanthropologist at George Washington University who was not involved in the new research.
And, given the nature of the fossil record and paleoanthropology, Dr. Wood says the implications of the new discoveries are not surprising. “It’s inevitable that the story changes by expanding the geography and increasing the time-depth” of our species’ history, he says.
But, Wood says, with this new research, “we're not as ignorant as we were before it was published.”