Fossils unearthed in Morocco from a little-understood period of human evolution may help scientists resolve a long-standing mystery: Who came before us?

Three jawbones, including one from a child, teeth, vertebrae and a femur were unearthed from a cave known as Grotte à Hominidés in Thomas Quarry in Casablanca, Morocco, dating back 773,000 years. They are intriguing to scientists because they are the first hominin fossils from this period to have been discovered in Africa.

“There are a lot of fossil hominins in Africa until about a million years ago, but then after that there is a jump to around 500,000 years ago, and in this gap we have almost nothing,” said Jean-Jacques Hublin, an author of the study that published Wednesday in the scientific journal Nature.

“It is extremely exciting to have fossils right in the middle of this gap,” added Hublin, a paleoanthropologist at Collège de France and the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

CT scans and analysis of the fossils’ features revealed an ancestor who had a “mosaic” of primitive and more evolved features. For example, it didn’t have a defined chin, unlike Homo sapiens, but the teeth and other dental features were quite similar to those of our own species and Neanderthals.

Most of the fossils were unearthed in 2008 and 2009, but they were definitively dated much more recently, Hublin noted, using a technique known as paleomagnetism, which detects the geological signature of a reversal of the Earth’s magnetic field in certain minerals with magnetic properties.

The strength of Earth’s magnetic field fluctuates, and, at times, the magnetic north and south poles have flipped. The research team found that the layer where the fossils were found coincided with the Matuyama-Brunhes transition, a well-known chronological marker that dates to 773,000 years ago and was the most recent major polar reversal.

Study coauthor Serena Perini, a geologist and paleomagnetist at Italy’s University of Milan, said in a statement the technique allowed the team to “anchor the presence of these hominins within an exceptionally precise chronological framework.”

The world’s earliest known Homo sapiens remains have also been found in Morocco at a site known as Jebel Irhoud, and they date to 400,000 years ago. However, Hublin said it would be incorrect to regard this region as the exact place our species emerged. More likely, it was a result of geological conditions in the region that allow fossils to be preserved particularly well.

Hublin noted that the cave these individuals called home would have been a dangerous place. The leg bone was covered in bite marks from a predator, most likely a hyena, and there was much evidence that carnivores occupied the cave.

‘Elusive figure’

The newly described fossils are important because they shed light on the ancestral species of the three types of human that lived most recently: Neanderthals, Denisovans and, of course, Homo sapiens, the only surviving human species.

Neanderthals and Denisovans are thought to have gone extinct around 40,000 years ago, although the timing is less clear for Denisovans, a shadowy population first identified in 2010.

The last common ancestor of the three human groups — sometimes dubbed ancestor x — is an “elusive figure,” according to Antonio Rosas, a researcher in the department of paleobiology at the National Museum of Natural Sciences in Madrid.

“Debate over which fossils might represent this crucial evolutionary node persists, and correctly identifying this ancestor is essential for understanding the directions of subsequent evolutionary change,” Rosas, who was not involved in the study, wrote in a commentary published alongside the new research.

Genetic evidence suggested that this ancestor lived around 550,000 to 765,000 years ago before splitting into three separate sister species, the study noted, but it’s not clear what the ancestral species was or where it lived.

Candidates include Homo antecessor, a group of fossils found in a cave in Atapuerca, Spain, that dates to roughly the same time as the Moroccan fossils, and a species known as Homo heidelbergensis, fossils of which have been found in Africa and Eurasia.

While the researchers stopped short of assigning the Moroccan fossils a formal scientific name, Hublin said the remains resembled another species called Homo erectus but also appeared to be close ancestors of modern humans.

“The question, then, becomes whether populations of Homo erectus directly gave rise to everything, including humans, Neanderthals, and Denisovans, or whether there is a traceable lineage with observable changes along the way,” said Ryan McRae, a paleoanthropologist at the Smithsonian National Museum of Natural History, who wasn’t involved in the research, via email.

Carrie Mongle, an assistant professor in the department of anthropology at Stony Brook University, said the new research emphasized the importance of Africa for understanding the emergence of modern humans.

“Any hominin fossil from this critical time period makes for an exciting new window into human evolution,” said Mongle, who also wasn’t a study author, via email.

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