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The discovery reinforced an idea that was emerging at the time: that there was not a single line of early humans, but multiple lineages. It was already known that three other species were living in Africa around the same time: H. habilis, H. erectus and Paranthropus boisei. H. rudolfensis added to this diversity.

Later finds from Koobi Fora suggest that the three Homo species coexisted between 1.78 and 1.98 million years ago.

H. erectus are thought to be our direct ancestors. They were the first hominins to migrate out of Africa, spreading into Europe and Asia.

but a 2013 study suggested that H. erectus had evolved the ability to throw.

This may have given H. erectus the drive they needed to become more social.There is some evidence that they shared information and worked in teams.

In the summer of 2015, researchers announced the discovery of the oldest known stone tools, dating to 3.3 million years ago. It had been assumed that only Homo species could make stone tools, but the tools were older than any known Homo fossils, suggesting that older species like A. afarensis or K. platyops could also make stone tools.

a 3.2-million-year-old fossil Australopithecus afarensis, nicknamed "Lucy". Lucy's species was immediately hailed as a key contender for our direct ancestor.

There was a single lineage descending from apes through to Lucy, "and then out pops our immediate ancestors". To her, that did not make

sense.

Her team found fossils on the western shore of lake that demonstrated there was "diversity at the age of Lucy".

A. anamensis. This was the oldest species known from Lake Turkana, having lived about four million years ago.

another new species called Kenyanthropus platyopus, or "flat-faced man". This species lived 3.5 million years ago, when other members of Lucy's species also roamed.

"the common ancestor" of Homo, and largely killed off the idea that humans evolved on a single line.

The earliest known Acheulean hand-axes were discovered near Lake Turkana in 2011. They are 1.76 million years old and were probably made by H. erectus.

There was thought to be a "substantial link between the emergence of humans (Homo) and the emergence of technology", says de la Torre, but it now seems this is not necessarily true.

Lake Turkana has played a pivotal role in our understanding of human evolution. But that is not to say the area was particularly significant for the early humans themselves.

Lake Turkana was simply an ideal place for fossils to be preserved, says Spoor. "That doesn't mean human evolution doesn't happen everywhere else in Africa."

It allows us to see multiple species that lived millions of years apart, and compare them.

“The evolution of cooperation required out-group hatred. Which is really sad,” said Nicholas Christakis, a Yale sociologist and author of “Blueprint: The Evolutionary Origins of a Good Society.”

No researcher argues that human nature is the sole, or even the primary, cause of today’s polarization. But savvy political operatives can exploit, leverage and encourage it. And those operatives are learning from their triumphs in divide-and-conquer politics.

“We wouldn’t have civilizations if we didn’t create groups. We are designed to form groups, and the only way to define a group is there has to be someone who’s not in it,” Mason said.

Experiments have revealed that “children as young as two will prefer other children randomly assigned to the same T-shirt color,”

enmity and derision can arise independently of any rational reason for it.

Mason and Christakis point to a famous-among-academics experiment from 1954. Social psychologist Muzafer Sherif took 22 Boy Scouts and separated them into two groups camping at Robbers Cave State Park in Oklahoma. Only after a week did they learn that there was another group at the far end of the campground.

What they did next fascinated the research team. Each group developed irrational contempt for the other. The boys in the other group were seen not just as rivals, but as fundamentally flawed human beings. Only when the two groups were asked to work together to solve a common problem did they warm up to one another.

And because many more districts are now deeply red or blue, rather than a mix of constituencies, House members have fewer reasons to adopt moderate positions.

“Homo sapiens is a social species; group affiliation is essential to our sense of self. Individuals instinctively think of themselves as representing broad socioeconomic and cultural categories rather than as distinctive packages of traits,”

Here’s where psychology gives way to political science. The American political system may cultivate “out-group” hatred, as academics put it. One of the scarce resources in this country is political power at the highest levels of government. The country has no parliamentary system in which multiple parties form governing coalitions.

Add to this fact the redistricting that ensures there are fewer truly competitive congressional races. The two parties have inexorably moved further apart ideologically, and leaders are more likely to be punished — “primaried” — if they reach across the aisle.

Shanto Iyengar, a Stanford political psychologist who coined the term “affective polarization,” explained in a 2018 paper why people typically identify with a group.

Human nature hasn’t changed, but technology has. The fragmentation of the media has made it easier to gather information in an echo chamber, Iyengar said. He calls this “sorting.” Not only do people cluster around specific beliefs or ideas, they physically cluster, moving to neighborhoods where residents are likely to look like them and think like them.

Partisan clustering has increased even within households. In 1965, Iyengar said, only about 60 percent of married couples had the same party registration. Today, the figure is greater than 85 percent

Asked in the summer of 2022 if they agree or disagree that members of the other party “lack the traits to be considered fully human — they behave like animals,” about 30 percent in both parties agreed, Mason’s research shows.

Research shows that affective polarization is intensifying across the political spectrum. Recent survey data revealed that more than half of Republicans and Democrats view the other party as “a threat,” and nearly as many agree with the description of the other party as “evil,” Mason said.

A recent paper published in the journal Science argued that the three core ingredients of political sectarianism are “othering, aversion, and moralization.” Trump has mastered that recipe. He activates emotional responses in his followers by telling them that they are threatened.