The life and death of the human species

How long have humans existed?

The earliest known fossils of Homo sapiens come from Morocco and are 315,000 years old. However, Homo sapiens probably lived in Africa earlier than this, maybe as long as 500,000 years ago.

Really though, the answer depends on what you mean by ‘human’. We can expand the definition of ‘human’ beyond our own species, Homo sapiens, to include all those extinct species that are more closely related to us than to our next-closest living relative, the chimpanzee. If we do that, humans have existed for around seven million years. 

The earliest species on the human lineage that we know about is Sahelanthropus tchadensis. This lived in Chad, in central Africa, about seven million years ago. It walked upright but probably looked and behaved much like a modern chimp. Other early human relatives that would seem more ape-like to us included Australopithecus africanus, Australopithecus afarensis (‘Lucy’) and Paranthropus robustus. All these creatures lived in Africa. The earliest of our extinct relatives that looked similar to a modern human was Homo erectus. This evolved in Africa more than two million years ago. It was the first member of the human family to leave Africa and spread throughout Europe and Asia. It evolved into many different forms including Neanderthals; the Denisovan yetis of Tibet; the hobbit Homo floresiensis; Homo antecessor, Homo naledi, the dragon-man Homo longi and many others – including us. Homo sapiens is the only member of the human family that still exists. All the others are extinct.

‘When will humans go extinct? This is a hard question to answer definitively. But humans will go extinct at some point, because extinction is what happens to all species, and there are signs that humans will go extinct soon, in geological terms.’

When will humans go extinct?

In my book The Decline and Fall of the Human Empire I predict that humans will go extinct within the next 10,000 years. But this is a hard question to answer definitively. Nobody can predict exactly when humans will go extinct. However, humans will go extinct at some point, because extinction is what happens to all species, and there are signs that humans will go extinct soon, in geological terms.

How will humans go extinct?

There are lots of answers to the question of how humans will go extinct. Some are chance events such as a large asteroid hitting the earth; a worldwide nuclear war; invasion of aliens from outer space; or takeover by some human technological development such as Artificial Intelligence (AI). Even with none of these things, humans will go extinct soon because all modern humans are genetically very similar, making us very susceptible to disease; a rapid decline in fertility; over-exploitation of the Earth’s resources; and climate change.

Did humans nearly go extinct in the past?

Yes, possibly many times. For almost all of human existence, humans have been very rare. Only since the invention of agriculture about 10,000 years ago have humans existed in large populations. That is when people started to live in villages, towns and cities. Today, about half of all humans live in cities, and this proportion will increase. For ninety-nine percent of the existence of Homo sapiens, humans were hunter-gatherers and lived in very small family groups.

‘Today, there is more genetic variation in one troupe of chimpanzees in West Africa than in the whole human population.’

The most risky time for humans was between 900,000 and 800,000 years ago, when there were never more than around 1,000 breeding humans at any one time. When the number of individuals is as low as this, or lower, extinction becomes a real possibility, whether from genetic disabilities caused by inbreeding, or localised chance events. All modern humans descend from this tiny group. A consequence of such genetic ‘bottlenecks’ is that modern humans are all genetically very similar. This is because they all descend from the very small pool of people that survived the period when the population was very small. Today, there is more variation in one troupe of chimpanzees in West Africa than in the whole human population. This is called the ‘Founder Effect’.

Will the human population ever stop growing?

Yes. The human population will stop growing in the 2060s. Ever since the invention of agriculture 10,000 years ago, the human population has been growing exponentially. Today there are more than eight billion people. The population is still growing but will reach its peak in the third quarter of this century, around the 2060s. At that time there will be ten to elevent billion people. After that the population will shrink very rapidly. By 2100 there could be eight billion (the same as today) and by 2300, as few as one billion.

Why will the human population stop growing?

The human population will stop growing for many reasons. The most obvious is a massive decline in human fertility. In the 1960s the rate of human population growth reached its peak, at around 2.3 percent a year. Today it is less than 1 percent. Human fertility in almost all countries is declining. This is measured by the Total Fertility Rate, or TFR. This is the number of children a woman must have in her lifetime. To keep the population stable, the TFR is 2.0 – two parents, two children. The population neither increases nor decreases. Actually, the TFR for stability is a little more than this, about 2.1, to compensate for various factors, such as the fact that slightly more boys are born than girls. Today, the TFR in nearly all countries is well below 2.1. Fewer people are being born than are dying. Populations are ageing.

‘Homo sapiens has now outgrown the ability of the Earth to sustain it. Resources are more expensive and harder to obtain; employment is scarcer and less rewarding; young people can no longer to afford to buy or rent a home in which they can raise children. All this is made worse by climate change.’

There may be many reasons for the decline in fertility. One is the ‘demographic transition’. It used to be that people had many children, expecting that most would die in infancy. Now, people have few children, expecting that they will survive. But people are having fewer children now, too few to sustain a population, except by immigration. Another factor is a massive decline in male reproductive health. Men, even if young and otherwise healthy, produce fewer sperm than they used to. Nobody knows why. It could be because of pollution, overcrowding or stress.

All this could be related to the environment. Homo sapiens has now outgrown the ability of the Earth to sustain it. The global economy has not grown appreciably for twenty-five years. Resources are more expensive and harder to obtain; employment is scarcer and less rewarding; young people can no longer to afford to buy or rent a home in which they can raise children. All this is made worse by climate change, which itself leads to scarcity, disruption in the distribution of resources, and conflict.

Will humans colonise space?

Maybe, but they will have to do it in the next 200 years if it is to succeed. By 2300, the human population on Earth will have shrunk so much that the pool of technologically educated human minds will be too small to engage in such large-scale projects.

‘If we are to save the human species from extinction, we must start to think seriously about long-term human space colonisation.’

We have a long way to go. Only twelve people have ever walked on the Moon – all healthy, educated men. Although around 400 people have been in space, only a few have gone above the Earth’s magnetosphere that protects people from damaging radiation. No woman has ever gone into deep space. Nobody has become pregnant or given birth in space. We know nothing about how people might raise children in space. Apart from knowing little about space medicine, the technologies we will need to make space colonisation a success, such as creating truly self-sustaining habitats, or creating food entirely artificially, are in their infancy. 

There are also legal and diplomatic problems that will need to be overcome. The Outer Space Treaty forbids any human from fencing off any region of space and creating a new, independent polity. Infractions of this might lead to lethal conflicts here on Earth. If we are to save the human species from extinction, we must start to think seriously about long-term human space colonisation.

From the deep palaeolithic past to our dramatic rise from one species of human among many to the most dominant animal to ever live on Earth, Henry Gee explains how we arrived just ten thousand years from our likely extinction. Drawing on a dazzling array of the latest scientific research, Gee tells the extraordinary story of humanity with characteristic warmth and wit, and suggests how our exceptional species might avoid its tragic fate.