A primer on early human evolution

The story of human origins is a messy one. Each bone fragment that’s unearthed or ancient genome that’s decoded adds a new piece to the puzzle – and it doesn’t necessarily make the picture any clearer. “Human evolution is not a line of cartoons from a bent-over chimpanzee to a modern human,” says Fred Spoor, a palaeoanthropologist from University College, London. “It’s a complex business.”

While Spoor and his colleagues revel in the increasingly bushy family tree that is emerging, the rest of us can be left up the proverbial tree.

So, here’s what we do – and don’t – know about where we came from.

The birth of Homo

Modern humans, Homo sapiens, are a single twig on a branch of the evolutionary tree that reaches back some seven million years, to when we split from our closest living relatives, the chimpanzees and bonobos.

Tracing a direct line of ancestry back along this branch is difficult because the fossil record is a patchy mosaic of incomplete skeletons. Few early humans died at the right time and place for their remains to be preserved.

Entire species probably became extinct without leaving a single toe bone for us to dig up in the smattering of places we are looking. And species that have been discovered are just as likely ancient “cousins” – offshoots of the branch leading to us – as they are great, great, great, etc. grandparents.

The fossil record between two and three million years ago – when our oldest Homo ancestors emerged – is particularly sparse, says Spoor, making it “one of the least understood parts of human evolution.”

Our earliest Homo ancestors most likely descended from Australopithecus afarensis, best known for the 3.2-million-year-old “Lucy” fossil found in Ethiopia’s Afar region.

A jawbone found in Ledi-Geraru, also in the Afar – marks the debut of Homo in the fossil record at 2.8 million years ago. By the time the fossil record begins to pick up around two million years ago, there were at least three early Homo species loping around Eastern Africa – Homo habilis, Homo rudolfensis, and Homo erectus.

What drove this early radiation is not clear, although many point to a changing climate – a common precursor to species booms. As East Africa dried around 2.6 million years ago – so the story goes – forests retreated and were replaced with open savannahs, terrain that would have favoured creatures that could run on two legs and fashion a stone tool to hunt game.

Homo habilis, with its prominent snout, was the most primitive looking of the three. But its larger skull, along with its discovery in 1964 in sediments that also contained the oldest known stone tools at the time – hence the moniker “handy man” – reveals Homo habilis to be an advanced species compared to its australopithecine ancestors. (Although stone tools dating back to 3.3 million years have since been found, and were probably not made by a Homo species at all).

For a long time, Homo habilis fit the bill as one of our direct relatives. But recent work, comparing the skulls of Homo habilis with other early Homo fossils suggests that this wasn’t the case. Homo habilis was an early side-branch, possibly splitting off from the 2.8-million-year-old Ledi-Geraru species.

Homo rudolfensis was a later offshoot, while Homo erectus – as we shall see – has assumed a central position in the story of human evolution over the last two million years.

All Homo species that crop up later than two million years ago – including us – are thought to have Homo erectus as their forebear.

Homo erectus and our first forays out of Africa

Homo erectus – “upright man” – was the first to bear more than a passing resemblance to modern humans. It was tall – up to 185 cm – with long legs, short arms, and a larger brain than its predecessors. It made hand axes and mastered the use of fire.

Homo erectus was also the first to venture out from its African homeland. Early Homo erectus fossils (sometimes referred to as Homo ergaster) first appeared in East Africa around two million years ago. By 1.8 million years ago, the species had migrated to modern-day Georgia, at the border between Eastern Europe and Western Asia.

From here it continued its march eastwards into mainland Southeast Asia (as in “Peking man”) and on to the islands of Indonesian (as in “Java man”) by 1.5 million years ago, where it persisted until as late as 100,000 years ago.

In Asia, Homo erectus gave rise to at least one species: Homo floresiensis, a dwarfed early human that lived on the Indonesian island of Flores 60–100,000 years ago. It is also known as “Flores Man” or “the Hobbit” due to its diminutive size.

Older floresiensis-like fossils, also on Flores, date to 700,000 years ago and stone tools on the island to one million years ago. It’s thought that the isolated outpost provided the perfect conditions for the island dwellers to shrink down to their petite size – just a metre in height – from their much larger Homo erectus ancestors.

Whether Homo erectus made it into Western Europe is unclear – certainly no fossils have been found to suggest they did. But by about 1.2 million years ago, a distinct species of early humans – Homo antecessor – was living on the Iberian Peninsula in modern-day Spain, perhaps arriving in a separate wave of migration from a predecessor in Africa.

Meanwhile, Homo erectus was busy colonising much of the African continent. Homo naledi, whose as-yet-undated remains were plucked from a cave system in South Africa, is one possible descendant.

Homo erectus was also spawning lineages that definitely did traipse into Western Europe, and others that would go on to become the globetrotting modern humans.

One of these descendants – Homo heidelbergensis – was the second species after Homo erectus to migrate out of Africa after it evolved (probably first as Homo rhodesiensis). It’s possible that it trekked east into Asia – a skull found in China is arguably from the species. But most remains show it spreading north and west into the colder climes of Europe, including Great Britain, thanks in part to its stocky build.

Homo heidelbergensis was a skilled hunter – it made spears to fell large game such as horses and elephants – and was the first to build simple shelters.

Modern humans and our contemporaries

Two important lineages come from Homo heidelbergensis: an African lineage leading to modern humans, and a European line that leads to Neanderthals (Homo neanderthalensis), Denisovans and possibly other species that were contemporaries of early Homo sapiens.

Neanderthals were the first on the scene: the oldest remains put their emergence back at around half a million years ago. From their European homeland, they moved east as far as Siberia in Central Asia, but never crossed into Africa.

Neanderthals were a sophisticated bunch. They hunted large game, plucked fish, molluscs and dolphins from the seas, and possibly sewed rudimentary animal-hide clothing using large bone needles. They also constructed mysterious rock structures in underground caves, but no one knows why.

Ultimately, however, Neanderthals became extinct around 40,000 years ago, although they may have held on as late as 28,000 years ago in southern Spain.

Homo sapiens, like our Homo heidelbergensis and Homo erectus ancestors before us, evolved in Africa – around 180,000 years ago – and then travelled north into Eurasia. Most current accounts, drawn from the fossil record and studies of both ancient and modern human DNA, suggest that they did this not once, but twice.

The first migration had Homo sapiens reaching the Middle East 100,000 years ago, and China as early as 80,000 years ago.

It’s not known how long any of these first migrants persisted, but by the time a second wave of Homo sapiens left Africa around 60,000 years ago, the original migrants had probably either retreated or become extinct.

Genetic studies that determine how living populations have interbred and mixed throughout our species’ history suggest that all people living today can trace their heritage to this second exodus from Africa.

Homo sapiens rapidly dispersed into Europe and Asia, reaching Australia by 55,000 years ago, and the Americas 13-16,000 years ago.

For decades, the demise of Neanderthals in Europe was chalked up to the arrival of the more competitive – or downright bloodthirsty – Homo sapiens. But the advent of ancient DNA analysis over the last decade has shaken up this tidy version of history. By cajoling ancient DNA from millennia-old fossils, palaeogeneticists have uncovered a more intriguing tale of interspecies trysts: humans with Neanderthals, Neanderthals with Denisovans, Denisovans with humans, and possibly others.

Sequencing of the Neanderthal genome from Croatian remains in 2010 revealed the first evidence that humans and Neanderthals mated. The first encounter probably occurred in the Middle East around 50,000 to 60,000 years ago, as Homo sapiens migrated out from Africa. All modern humans, bar Africans, still carry traces of Neanderthal DNA – usual around 1-2% of the genome.

Later matings in Eastern Europe and Asia have been proposed, and an earlier Middle Eastern encounter between Neanderthals and the first wave of human migrants also took place.

Humans also interbred with the Denisovans, a group known from limited remains – a finger bone and a few teeth – recovered from a Siberian cave and dating to around 100,000 years ago. (Neanderthals and Homo sapiens remains have also been found in the cave.)

Most of what we know of the Denisovans comes from ancient DNA gleaned from these remains. Modern Aboriginal Australians and Melanesians have the highest percentage of Denisovan DNA in their genome – some 2–4% – and modern Tibetans likely inherited their ability to cope with high altitudes from Denisovans.

This suggests that Denisovans could have lived throughout Asia. Indeed 10,000 to 14,000 year old fossils found in caves in southern China – the Red Deer Cave and Longlin people – may even turn out to be Denisovan.

The Denisovans weren’t a picky lot. They also interbred with Neanderthals, and a more archaic mystery species, which some speculate could be Homo erectus, or even Homo floresiensis. There are ongoing efforts to extract DNA from these species to see if this is the case.

“I’d have loved to have been on the planet 60,000 years ago,” says archaeological scientist Richard “Bert” Roberts from the University of Wollongong, who helped date the Hobbit remains. “We used to have a fabulous time, with all sorts of other humans running around the planet.”

However many times we and our extinct relatives did interbreed, these ancient DNA studies highlight how different modern times are to almost any other period in our pre-history. Whereas now we roam the world as the single living representative of the Homo line, all previous times have seen the world populated by multiple related – and often intermingling – species. Our once bushy family tree has been pruned back to a lone twig.

“What a boring state we’re now in with only one human species left on the planet,” says Roberts. “What a yawn!”

This article appeared in Cosmos 75 – Winter 2017 under the headline “Where we came from”