It can be hard enough to remember who your second cousin once removed is. So it’s not surprising that tracing back the family tree to work out what the earliest humans were up to hundreds of thousands of years ago is quite a challenge.
“Everyone’s looking for the earliest evidence for modern humans everywhere,” says Professor Sue O’Connor, an archaeologist at ANU. “There is quite a lot of research effort focused on this.”
But despite the input of archaeology, genetics, and other diverse fields of research, we’re still a long way from understanding how our mysterious ancestors got us to where we are today.
Out of Africa
The Out of Africa theory – the idea that the very earliest humans originated from Africa, where they left in a single migration to populate the globe – is the most widely accepted theory of our species’ geographical origin.
It is also perhaps the only widely accepted theory when it comes to retracing the steps of our ancestors.
Both the fossil record and DNA evidence places hominins (modern and extinct human species) in Africa. As for when, where, and why they ventured off elsewhere, the evidence is far less conclusive.
“Even the date of Out of Africa is quite unclear,” says O’Connor. “You see completely different dates quoted. Anything from 60-80,000 [years ago], you see 100,000, you see 120,000; you see things saying the DNA evidence suggests it may be double that. We’re not there yet.”
Methodological limitations and small sample sizes mean there is a considerable margin of error when it comes to determining an estimated time of departure, explains Dr Bastien Llamas from the Australian Centre for Ancient DNA at the University of Adelaide.
“A lot of people have focused on specific dates – when did humans leave Africa?” Llamas says. “It goes anywhere between 70 or 50 thousand years ago, to 200,000 years ago. Which leaves a large margin of uncertainty!”
Llamas adds that as more data is collected from around the world, the picture becomes clearer. “It’s not only when humans left Africa, but also other migrations cross the globe: When did they reach the Americas, Australia, all the migrations across Asia – all these things become clearer.”
Searching for the pathway
But the precise routes traversed by our oldest relatives are still being debated.
“The presumption is that they moved through the Arabian Peninsula, then to South Asia and then South East Asia,” says archaeologist Professor Peter Hiscock from the University of Sydney. “But we don’t actually know the pathway by which they moved.”
“Modern Aboriginal people have some genes that come from another form of hominin, which is found in North Central Asia,” Hiscock adds, explaining that DNA evidence shows Aboriginal ancestors mixed and interbred with Denisovans, an extinct human species whose only remains so far have been found in Siberia.
“It’s possible that on their journey, they actually went northwards, maybe across the top of India. But we really don’t know,” Hiscock adds.
Without knowing exactly when humans left Africa, or where they went from there, it’s almost impossible to understand why they upped and left in the first place.
One possibility is environmental fluctuations that shifted African-type habitats into the Middle East, says Hiscock. “People may have actually just moved out of Africa without knowing it, following the ecosystems they were familiar with,” he explains.
“I think you’ve got to look at it as a sort of incremental thing that isn’t a deliberate colonisation,” says O’Connor. “It’s not like everyone’s going ‘oh wow, we can get out of here now!’”
A messy route
“We have a very, very slim actual human fossil record,” says O’Connor.
According to her, the fossil evidence for human migration routes is “quite problematic.”
In Australia, fossils dating back 50,000 years are well preserved, but in most parts of the world a lot of the evidence is based on stone artefacts rather than human remains.
Ancient DNA can provide more clues about where ancient hominins went, and whom they encountered on their travels. But it’s difficult to find well-preserved samples with enough DNA material to work on, says Llamas – and you have to be extremely careful not to contaminate it with traces of other living matter.
“We wear full body suits, with gloves and face masks, and the facility itself is regularly bleached to remove any trace of DNA,” he adds.
This careful DNA research is building our understanding of our ancestors’ migration routes and breeding habits (such as with Denisovans or Neanderthals). But just as archaeology suffers from a lack of fossils, genetic research needs more ancient DNA samples to paint a clearer picture.
Bodies of evidence
Along with improved technology for generating DNA data, Llamas says genetics needs to work with other research areas to put it all together.
“Of course archaeology, but also arts, linguistics, anthropology, anatomy … there’s a lot of bodies of evidence that can be gathered and grouped together to have a holistic view of human evolution,” he says.
Modern human scientists are sure to keep working to understand our true origin story.
“I think it’s just the human fascination with humans,” says O’Connor. “When did they develop, did they have speech, did Neanderthals have speech? Where did people first invent this? Did other hominins have culture like us? We just have a fascination.”
“We’re the most navel-gazing of all species, aren’t we?”