How on Earth did we get here? No, not whatever political or environmental or existential crisis just sprung into your mind. How did we, as humans, end up where we are today?
Discoveries pushing back milestones in human development happen all the time. That can get confusing. So here’s a very basic (but not brief) cheat sheet of the current research and findings—for the next time the origin of humanity comes up during happy hour.
How did we spread around the world?
The oldest fossils of humans and their relatives show up in Africa, where they’ve been preserved for thousands or even millions of years. But while Africa has earned its popular designation as the Cradle of Humankind, it's not as if our ancient ancestors stayed confined to the nursery for long.
Researchers think that modern humans and other hominins left via the Middle East, taking advantage of the land bridge between Eurasia and Africa. But how much connection there was between these different areas is still a matter for debate.
In a new study in Science, archeologist Johannes Krause and colleagues looked at 15,000 year old bones from Morocco to see where they might have come from. They were associated with a group that researchers thought might have European roots, potentially crossing back into Africa from Europe via the Strait of Gibraltar during the last Ice Age. But that story wasn’t holding up as we got more evidence.
“Archeologists recently showed that the technology found at the site doesn’t look European. It looks more like the Near Eastern technologies, and that has raised some interesting questions," says Krause. "How does that Near Eastern technology come to Western Africa?” So he and his team took DNA samples to see where the people buried in those sites were really from.
“To our surprise we found that ⅔ of their ancestry is indeed Near Eastern, like the Levantine hunter gatherers, the so-called Natufians who were living in the Near East before agriculture emerged in this region," he says. "The missing third, surprisingly, was from an African population... we have actually no clue where it comes from. It’s related to East Africa, West Africa, and Southern Africa. The wide mixture of African relatedness indicates to us that this African component is quite old, and is probably a ghost population—a population of Sub-Saharan Africans that is not present any more.”
For Krause, the data shows that long before the Romans and Greeks and Phoenicians took a stab at sailing across the Mediterranean, other groups of humans were mixing and moving, just like we’ve always done.
“We’re not stable, we’re not like, ‘my ancestors have been in this valley for 20,000 years’—that will never happen," Krause explains. "People have moved all over the place all the time.”
“That’s important information for a lot of people today," he adds, "with a new form of nationalism looming in a lot of places of the world. What we see here is that, at least in the past, people have been extremely mobile. If people want to now build walls and borders and force people to stay in a certain place, then that is the exception. That is not the rule.”
Other evidence goes back even farther into our past.
Last month, in a paper presented in Science, researchers led by Israel Hershkovitz announced the discovery of bones and tools at a site in Israel called Misliya Cave, Mount Carmel. They dated the artifacts and remains to around 180,000 years ago, pushing back the time that members of our species left Africa by 50,000 to 85,000 years.
"Misliya is an exciting discovery," said study co-author and anthropologist Rolf Quam in a press release. "It provides the clearest evidence yet that our ancestors first migrated out of Africa much earlier than we previously believed. It also means that modern humans were potentially meeting and interacting during a longer period of time with other archaic human groups, providing more opportunity for cultural and biological exchanges."
Let’s talk tools.
Speaking of cultural and biological exchanges: in a new set of papers also published in Science this week, researchers focused on a technological transition in Kenya. Rocks and tools are some of the most durable parts of the archaeological record, and these showed a huge shift away from the hand axes that dominated hundreds of thousands of years of early human history. Instead, they were using smaller, more precisely crafted stone tools, made from obsidian found miles away, and decorating items with pigments.
“That’s where there’s a similarity to technology in recent times; things start out big and clunky and they get small and portable," says Richard Potts, head of the Smithsonian's Human Origins Program and a co-author of the papers. "The history technology has been the same ever since.”
But why the shift away from our clunky, durable hand axes?
“One of the things we see is that around 500,000 years ago in the rift valley of southern Kenya, all hell breaks loose. There's faulting that occurs, and earthquake activity was moving the landscape up and down. The climate record shows there is a stronger degree of oscillation between wet and dry. That would have disrupted the predictability of food and water, for those early people,” Potts says. “It’s exactly under those conditions that almost any organism—but especially a hunter-gatherer human, even an early one—would begin to expand geography of obtaining food or obtaining resources. It's under those conditions that you begin to run into other groups of hominins and you become aware of resources beyond your usual boundaries.”
Potts thinks that the environmental changes encouraged early trade and social networks to start developing between these people around 320,000 years ago. But we still don't know exactly who these people were.
“We wish we knew,” Potts laughs. “Would they have been pretty archaic looking, more like the species known as Homo heidelbergensis that we have evidence of back to 600,000 years ago in Eastern Africa, or did they look more like us, homo sapiens? We don’t know the answer to that.”
One way that we can start painting a better picture of our ancient relatives is by looking into DNA. Scientists have tested and categorized thousands of remains in ways that wouldn’t have been possible five years ago, especially not with remains from Africa.
“Africa is a really warm place, and here’s a correlation between temperature and preservation,” Krause says. “It's like a piece of meat: you keep in the fridge, you don’t put it on the table, because it gets smelly after two days or so. And what we’re analyzing used to be a piece of meat.”
But advances in genetic testing, and a better understanding of what parts of our body best preserve DNA (the inner ear bones, according to Krause) mean that we’re getting more information than ever about where we came from.
“It’s been speculated that people left Africa 45,000 years ago to become Europeans, Asians, Australians, Native Americans, and so forth," he says. "But was there then migration to back to Africa? Was there gene flow between Africa and other parts of the world? That’s [a question we've] been trying to address through anthropology and archeology, but now we have this beautiful new tool called genetics and ancient DNA.”
“Now we can measure it directly. We don’t have to look at shards or ceramic or ancient bones. We can look at ancient DNA, and we can measure how much genetic closeness or mixture we can see between different populations. And that’s really exciting,” Krause says.
How did our ancestors interact with other early human species?
Without a time machine, we’re never going to have a full picture of inter-species relationships in the Homo genus. But a growing pile of genetic evidence shows that we had a very close relationship with others.
Modern humans are all members of the species Homo sapiens. We’re the last hominins standing, having endured longer than other species and sub-species, including H. neanderthalensis, better known as neanderthals, perhaps our most famous ancient human relatives. While neanderthals didn’t persist to the present, many humans alive today do share many genes with neanderthals. Yes, those genes mean what you think they do: there definitely was some interbreeding.
Then there are the Denisovans, which we only know of from the genetic analysis of a fossilized pinky finger, but who likely interbred with us as well. A study published this week in Cell suggests it happened at least twice.
A lot has changed in the past few hundred thousand years, not in the least because we are now alone. Once upon a time, humans had a decent amount of company. Our species shared the world—and interacted with—all sorts of other humans for a very, very long period of time.
When did humans show up in the first place?
That’s one of the biggest questions in anthropology. And in a lot of cases, it comes down to where you want to draw the line. Did we become human when we started walking on two legs?
One of the earliest accepted members of the human family tree is Sahelanthropus tchadensis, which lived roughly around 6.5 million years ago and was likely bipedal.
But what if we become human when we started making tools?
Fast-forward a few million years, and Homo habilis arrives. H. habilis was one of the earliest members of our genus, and we see fossil evidence of this species dating to 2.4 million years ago in Africa. This was once thought to be one of the earliest species to make and use stone tools—hence the name, which means handy man—but older evidence of stone tool use has since emerged, and some researchers are reconsidering whether or not they should really be included in our genus, anyway.
We’re inching closer to our species. Early fossils of distinctly H. sapiens individuals have been found and dated to between 150,000 and 200,000 years ago, mostly in East Africa, but they could have shown up even earlier. Last year, researchers announced that tools and fossils found at Jebel Irhoud Morocco dated to roughly 315,000 years ago. These fossils share some features with Homo sapiens, enough to cause a debate as to whether or not they count as members of our species, or if they’re close precursors.
But one thing that’s important to remember is that while we can identify different species in the fossil record, the pace of evolution is way more messy than a chart in a textbook might have you believe.
“It’s not going to emerge as a package,” says Shara Bailey, who worked on the Jebel Irhoud site. “It’s not like all of a sudden you’re going to have people that look like you and me. You might have teeth that look modern, but backs of the skull that don’t.”
Potts (whose research focuses on a different part of the continent, but roughly the same time period as Bailey's) agrees. "We’re really dealing with a transition. We like putting things into categories, is it homo sapiens or not, but we’re really dealing with an evolutionary transition in the ways in which the those early humans looked," Potts says.
The full picture of our morphological history is still emerging. The discovery in Morocco may also expand the potential range of our ancestors (or at least, our relatives) beyond East Africa and across the continent.
And then there’s our tools, our innovation, our social structures, and all the other things that make us human today. That’s where the studies Potts worked on enter the picture again.
“I think what we’re dealing with are a suite of behaviors. Innovation in not only technology, but changes of social life with regard to this resource exchange, and evidence of more complex symbolic behavior, the ability to communicate with groups that are distant and that you might not see every day,” Potts says. “These are the behaviors that may have distinguished our gene pool and Homo sapiens from that of our ancestors.”
So what comes next? There’s a lot more of our human history yet to be excavated around the world, and a lot more tools available for scientists to use. There’s no reason to think the pace of discovery will slow any time soon.
“That's what science is about. We’re constantly working our way closer to whatever truth is out there," Bailey says. "So at any given time, we can only draw conclusions based on the evidence that we have. As we find more evidence, we have to be flexible enough to rethink our hypotheses."
When did life start on Earth?
This is one of the more hotly contested debates in biology. Researchers are still trying to figure out where and how life began, let alone the time period during which our planet shifted from a population of zero to Pop. 1+. When it comes to timing, the scale of the evidence that biologists are looking for is nearly impossible to comprehend. They’re searching for fossils that have not only managed to survive for billions of years, but that were left behind by lifeforms the size of bacteria. Given the fragility and size of microbes and the rate of change on our dynamic planet, it’s incredible that the first Earthlings left any physical traces at all.
But some examples do exist. Some of the oldest fossils many researchers agree on are microfossils from Australia that clock in at 3.43 billion years old. But other scientists have found fossils they claim are older. (For more, read Popular Science’s feature from our September/October 2017 issue: A new finding raises an old question: Where and when did life begin?.)
But fossils aren’t the only evidence we can use to scout out the origins of life. Chemical and geological analysis of rocks might indicate life going back 3.8 billion years, and maybe even as far back as 4.1 billion years.