Are Modern Humans Really Older Than We Thought?

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You and I might be older than we thought. Not us personally— us as in Homo sapiens, or anatomically modern humans. That’s a term that refers to someone whose bone structure falls within the range of humans alive today. Someone who wouldn’t look too out of place if you just saw them walking down the street. For a long time, fossil evidence for anatomically modern humans went back less than two hundred thousand years. But an international team of researchers, publishing this month in the journal Nature, have found what they’re calling early Homo sapiens fossils that are around three hundred thousand years old, at a site called Jebel Irhoud in Morocco.

This research has been making headlines as a scientific breakthrough, but it’s a little bit more complicated than that. What we can say for sure is that these papers are more evidence in an ongoing anthropological puzzle, and we should keep revising our ideas of exactly when, where, and how we first evolved. We know anatomically modern humans arose from archaic humans like Homo erectus in Africa. And the 200,000 year figure has been pretty persistent, but there have been hints that Homo sapiens is actually older than that. For one thing, a DNA study published last year suggests ancestral humans might have diverged genetically from ancestral Neanderthals more than 500,000 years ago. We’ve also found fossils older than 200,000 years that look like Homo sapiens, but they were too fragmentary to be sure.

Before these recent papers, the fossils from Jebel Irhoud didn’t seem like important evidence in this age debate. The site has yielded hominin fossils since 1960, but sloppy or inaccurate dating led researchers to think they were as young as 40,000 years. One estimate based on radioactive uranium and a technique called electron spin resonance put them at around 160,000 years old. Even that is kinda boring if you’re looking for human origins, so the site was consistently overlooked. But the current find involves new excavations and a dating technique that turned the clock back past 300,000 years. The excavations found new hominin fossils near flint tools, along with charcoal and burned mammal bones, which suggests they were probably using fire. Some of the tools were cracked as if they had been heated, and that opened them up to a dating technique known as thermoluminescence. Crystals of certain common minerals, like flint, which is a form of quartz, tend to trap electrons over time. Those electrons come from a source like sunlight or the natural radioactive decay of elements in the ground. When those crystals get heated up, the electrons get enough energy to escape. The crystals zero out, and start building up electrons again. When some early Homo sapiens let their tools get toasty, they reset the electron content of the quartz crystals. And because electrons build up at a steady rate, it creates an electron clock. The number of electrons is proportional to how long it’s been since the crystals were last heated. The researchers then heated the tools again, which forced the electrons out in the form of weak light. By measuring that light, they could tell how long it had been since those tools got left in the flames, and then use that to estimate the age of the fossils found in nearby rock. The number they came up with was around 315,000 years, plus or minus a few. And then they double-checked by doing some more radioactive uranium and electron spin resonance dating on a fossilized tooth, which gave an estimate of 286,000 years. So the fossil ages are pretty solid, but researchers don’t entirely agree about how much they shake up the picture of when and where anatomically modern humans emerged. Modern humans from the last 130,000 years have relatively small faces and globe-shaped braincases, which is the back part of your skull. The Jebel Irhoud hominins shared the small faces, but their braincases were more elongated than what you would see in people today. Earlier thinking had modern humans arising in a rapid evolutionary event 200,000 years ago, taking place somewhere in East Africa, in which our faces and braincases both changed from ancestral hominins. This research isn’t the first to question that thinking, but the mix of archaic and modern traits in these fossils might mean a couple of things. For one, the sudden emergence of anatomically modern humans is probably thanks to the fossil record, not a real event. The Jebel Irhoud fossils suggest a transitional form, in which humans evolved small faces early or even kept them from an archaic ancestor, and didn’t get the modern braincase until later. So even though it’s flashy and exciting to claim that these are the earliest Homo sapiens, it isn’t necessarily a clear-cut label. The paper authors say that these fossils are early Homo sapiens, and most scientists seem to agree that these are our direct ancestors, at the very least. Whether you consider them recent humans or not, this discovery also suggests that hominins like the Irhoud folks were living all over Africa. The study authors point to 260,000 year old fragmentary remains from South Africa, which their research confirms could plausibly be Homo sapiens. South Africa isn’t exactly an afternoon stroll from Morocco, and neither are terribly close to a hypothetical East African origin for modern humans. The researchers say all this evidence points to a pan-African view of human evolution, with no one site being the unique cradle of humanity. As is often the case in biology, it was more of a mess. A mess that’s elegant in its complexity, because it all fits together if you can find all the evidence, which we are still doing. And still reporting on, here on SciShow News! If you like this weekly update about what’s happening in the realms of science, you should check out news on SciShow Space, too. And don’t forget to go to youtube.com/scishow and subscribe.

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