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The Human Flood

floodLecture four of “The Cognitive Revolution” concerns the flood of homo sapiens all over the planet.

While doing this, they drove numerous other species to extinction. In Australia, up to 95% of all large animal species vanished. In America, 84 of 107 large mammal species disappeared. Altogether, about half of the large terrestrial mammals that populated Earth became extinct. How could a few million individuals who possessed no more than Stone Age technology have caused such devastation?

Dr. Yuval Noah Harari looks at theories of what happened to all many of the mammals and of the impact of our species on the planet. These are my notes of his video lectures.

To understand how sapiens became the most important factor in the ecology of planet Earth, long before the agricultural revolution, we need to examine the relations between Sapiens and the rest of the ecological system, how sapiens related to other animals and plants.

Crossing the Oceans

(Credit – ghoraba.blogspot)

One of the most important things to realize about the transformation of Homo Sapiens from insignificant to the most important animal is the geographical aspect, how they spread around the world. Until the rise of homo sapiens all human species, Neanderthals, Erectus and the others, lived exclusively on the Afro-Asian land mass that includes Africa, Europe, Asia, and the nearby islands. Ancient humans managed to reach some islands close to the shore  in two ways. First of all, some islands were reached simply by walking there in periods when, due to climatic conditions, the sea levels went down so there was a land bridge connecting the mainland to what later became an island. People walked there and got stuck on the island when the sea level rose again. Alternatively, some islands close to the shore were settled by humans who swam there, or hung onto tree trunks and improvised rafts and managed to cross short distances of open sea in this way. However, none of the archaic human species before Homo Sapiens had the ability to cross large expanses of open sea and to reach and settle the Continents of America, Australia or the remote islands like Madagascar, New Zealand or Hawaii.

African animals. (Credit – Wikimedia)

The sea barrier prevented not just humans but also  other Afro-Asian animals and plants from reaching this outer world and prevented Australian and American animals from going over to Africa or to Asia. For tens of millions of years there were different separated ecological systems. Animals evolved in each of these places and plants also evolved in each of these places. Kangaroos and the other marsupials were evolving in Australia, for tens of millions of years, without any connection with elephants, tigers and alligators, in Asia and Africa, they couldn’t cross the ocean, and reach from one place to the other.

Planet Earth was thus divided into several distinct ecosystems. One of the main processes of history is the unification of the entire planet, for the first time into a single ecological unit by Homo Sapians. They began to forge connections between the animals of Australia and those of Africa, Asia, and America by moving them from place to place. This began shortly after the connective revolution when sapiens acquired the technology, the organizational skills and perhaps even the vision necessary to breakout off the Afro-Asian landmass and begin to explore and settle these areas of the outer world.

Australian Animals. (Credit – Wikimedia)

The first breakout from the inner world of Afro-Asia to the outer world occurred about 45,000 years ago in Australia. Experts today are very hard pressed to explain these amazing feats. We know for certain that they reached Australia, because we found sapiens’ bones and tools but how they crossed over, from Indonesia to Australia, is very difficult to explain. Even though there are islands off Indonesia, Southeast Asia and Australia, between these islands, there are large stretches of open sea up to 150 kilometres wide. The questions we need to look at are how Sapiens could cross large, open stretches of sea and, more importantly, how they managed to adapt themselves, almost overnight, to a completely new ecological system.

The problem of settling is Australian was not confined to crossing the sea. It was equally difficult to adapt the conditions there. Sapiens who had lived in Southeast Asia and Indonesia for thousands of years knew the animals, plants and conditions there which was vital for survival. In Australia they suddenly encountered a completely new world with unfamiliar animals and plants. Hunting elephants is not the same as hunting kangaroos. Different mushrooms would be poisonous in Australian to those of Asia. Knowing how to treat an Indonesian scorpion bite would not necessarily help you to treat the bite of an Australian scorpion or spider. Sapiens needed not only to cross the sea but to adapt almost over a night to a completely new ecosystem.

First seafarers.

Click the image for a larger version (Credit – Wikitravel)

The most reasonable theory suggests that sapiens living in the islands of Indonesia, which are close to the mainland, developed the first seafaring societies. They learned how to build and manoeuvre ocean going vessels and became fishermen, long distance explorers, and traders. They became more and more used to making a living by going out to sea. These skills enabled the ancient Indonesians to reach and settle Australia. If this is correct then this was an unprecedented revolution in human abilities and in human lifestyle. Sapians, are basically African apes who have evolved for millions of years adapting to life on land and suddenly, within a few thousand years, to develop societies that live by building boats and going overseas and fishing and trading by sea is without precedent in the annals of life on earth.

(credit – dolphin-way.com)

We have many examples of sea creatures which evolved to become land creatures, and also vice versa. For example, the ancestors of dolphins and whales were land creatures. Over the years they evolved to live in the ocean. This took millions of years of evolution for the archaic dolphins, which were a kind of deer, or rhinoceros, that started to spend more and more time in rivers and lakes like Hippopotamus and then went into the ocean. It took just a few hundred years for these apes from the African Savannah, without changing their bodies, to develop seafaring societies based not on new genes but on new technological abilities and social skills. These skills enabled them to build boats, cross the sea, to fish and to learn the skills necessary to navigate the ocean, withstand storms and to swim and many other things. The journey of the first humans to Australia is not only remarkable from an evolutionary perspective, but also from an historical perspective. This journey is one of the most important events in history. It is at least as important as the journey of Christopher Columbus to America in 1492 and all the journeys of the Apollo 11 expedition to the moon. The journey of the first sapiens to Australia was the first time that any human species managed to leave the Afro Asian ecological system and settle in new ecological system. Indeed, it was the first time that any large, terrestrial mammal managed to cross over from Afro-Asia to Australia.

Effect on the animals

The Marsupial lion. (Credit – Wikipedia)

Of even greater importance than the crossing was what the human pioneers in Australia began to do in this new world. The moment that the first hunter gatherer set foot on the beach of Australia, was the moment that Homo sapiens climbed from the middle to the top rung of the food chain and became the deadliest specie in the annals of plant earth. Up until then sapiens had shown showed a remarkable ability to adapt to different environments with negligible impact upon the environment, and of moving into and adjusting to all kinds of habitats like deserts, swamps and frozen regions without really changing them. They changed the ways that they behaved. The first settlers of Australia began to transform the Australian ecosystem beyond recognition. Australia about 50,000 years ago was very, very different from the Australia that we know today. It was full of large and strange creatures including for example a giant Kangaroo which reached heights of two meters and weighted about 200 kilograms. Australia was also home back then to giant Koalas, flightless bird twice the size of ostriches, giant lizards that looked like dragons and giant snakes. There was also the marsupial lion, which was as massive as a modern tiger and was the

Diprotodon optatum (Credit – Wikimedia)

continent’s largest predator. The largest creature of all, in Australia, of those days, was the Diprotodon (Giant Wombat), which weighted about two and a half tons. Australia was full of big, strange creatures but within a few thousand years, of the arrival of Homo sapiens, almost all these giants vanished. 50,000 years ago, there were 24 Australian animal species that weighed more than 50 kilograms on average. Within a few thousand years of our arrival  twenty three of them were extinct. The only one that managed to survive is the red kangaroo.  At the same time a large number of smaller species also disappeared and food chains throughout the entire Australian ecosystem were completely changed and rearranged.

This was the most important transformation in the Australian ecosystems for millions of years. If you look at the history of the Australian ecosystem there are millions of years of continuity then 45,000 years ago, a big break, a big catastrophe in which, suddenly many species, especially the large species, disappear. Some scholars put the blame on climate change, but there is very strong evidence that implicates Homo sapiens in the catastrophe. We can’t be 100% sure, but it’s very likely that Homo sapiens did it. The big question is how a relatively small number of hunter gatherers 45,000 years ago with technology of the Stone Age managed to cause such a huge ecological catastrophe. There are several explanations that are not contradictory but mesh together quite nicely to give us a relatively complete picture of this ecological disaster.

What happened to Australian animals?

The first explanation is that large animals which were the primary victims of this Australian extinction bread slowly. Pregnancy was long, it took month and maybe years for a large animal to reproduce. The number of offspring per pregnancy was very low, usually just one among the very big animals and there were long breaks between pregnancies. Whereas rabbits breed quickly large animals have many years between one birth and the next. Consequently, even if sapiens killed just one every few months it would have been enough to cause the number of deaths in the local Diprotodon community for example to outnumber the births. Over a few generations, this was enough to drive them to extinction.

The second explanation is that in Australia sapiens had the element of surprise on their side which they didn’t have in Africa or Asia where, humans long before sapiens, evolved slowly. Gradually over hundreds of thousands of years, they acquired better skills and became better hunters. The animals that humans hunted like giraffes, bison and Elephants gradually learned to be wary or even afraid of humans. When the new mega predator Homo sapiens appeared on the Afro-Asian scene, the large animals already knew to keep their distance, that this ape was dangerous. In contrast, the Australian giants had no time to learn to run away from humans. According to the normal criteria of the ecological system, humans don’t come across as particularly dangerous, we don’t look very frightening. Compared to a tiger or to an alligator we don’t look very dangerous. We have much fewer muscles, can’t run as fast as a cheetah, we don’t have big teeth or strong jaws like a tiger. We don’t have any poison or sharp nails.  When the first humans arrived in Australia the big animals looked at creatures just like us and went back to munching leaves. Before the diprodotons and giant kangaroos and giant koalas had time to evolve respect or fear of humans, they became extinct. It took humans very short time to drive them to extinction.

Fire Agriculture (Credit – wmo newsletter)

In addition, there is the explanation of fire agriculture. Fire agriculture means, using fire to reshape the environment to fit your needs better. When Sapiens landed in Australia they were already expert in the use of fire agriculture. They used this to deliberately reshape the Australian ecosystem by burning vast areas of thickets and dense forests in which they had difficulty moving and hunting. This opened the way for the growth of open grasslands in which it was easier for them to move around. Open grasslands attracted more easily hunted game. We have interesting evidence for this from the ecological history of Australia for instance with regard to eucalyptus trees. Apparently, until about 45,000 years ago, eucalyptus trees were quite rare in Australia. They became very wide-spread only after sapians arrived and began to burn down large forests and woods. Eucalyptus trees are more resistant to fire than most other kinds of trees. They survived and even flourished despite the fires, and they became one of the dominant trees of Australia.

(Credit – globalcitizencorps.org)

A final explanation for how sapiens drove so many animals to extinction in Australia is that in addition to the danger of hunting, and fire agriculture, climate also played a significant. There was some kind of climate change that beset Australia about 45 thousand years ago. It destabilized the ecological system, and made it particularly vulnerable. Under normal circumstances, the ecological system could have recuperated. Climate change had happened many times previously. The diprodoton, for example lived in Australia for more than a million years and there were many events of climate change and, difficult climatic conditions, which the diprodoton managed to survive. The difference 45,000 years ago was that at the same time as climate change there was the sapien’s invasion.  The combination of climate change with sapiens’ activates of hunting and use of fire attacked the animals from two different direction, at the same time.  They couldn’t find a good strategy to survive both the changing, climatic conditions and the new threat of sapiens invaders.

Lacking further evidence, it’s hard to decide which of these factors is the most important. Whether it was the element of surprise, hunting, fire agriculture, or climate we have enough evidence to be quite convinced that if Homo sapiens had never reached Australia the climate change by itself would not have been enough to cause such mass extinction. If sapiens had never reached Australia it would probably still have been populated with giant kangaroos, koalas and diprodotons. The extinction of the Australian big animals was therefore the first significant mark that our specie Homo sapiens, left on planet Earth.

A thought experiment

(credit – hplusmagazine.com)

Imagine that 70,000 years ago some mysterious epidemic killed all human species including Homo sapiens.  50 million years from now rats become the dominant life form on earth evolving to become very intelligent so there are rat scientists and rat professors who go about writing the history of life on Earth. Would these intelligent rats have any reason to give attention to the long extinct humans?  Probably not. They would have no reason to note the fact that, 50 million years ago there lived, in Africa and Asia, these kind of apes who had large brains and made stone tools and used fire, and then became extinct In a big epidemic.  Humans, however interesting they were, did nothing important in the ecosystem until 70,000 years ago.

Now imagine that this terrible epidemic erupted only 40,000 years ago so humans had a chance to reach Australia and kill all the big animals there. Now the rat professors of the future would have reason to pay humans much more attention. They would write in their articles and books that once upon a time, 50 million years ago, there lived this species of apes with large brains and stone tools and fire and they spread from Africa, and the one big thing that they did was to completely change the ecology of Australia before being wiped out by this epidemic.

This was the first big thing that our species did in the world to destroy the large animals of Australia. Subsequent they were responsible for similar and even larger ecological disasters in many other parts of the world.

The second disaster

(Credit – Option Canada)

The second really important thing that humans did was an even larger ecological disaster this time in America. Homo sapiens were the first and only human species to reach the Western Hemisphere, the continent of America, arriving there about 16,000 years ago. That is around, 14,000 BC.

The first Americans arrived on foot, which they could do because at the time sea levels were low enough, so that a land bridge connected North Eastern Siberia with North Western Alaska. Sapiens arrived in Alaska probably in pursuit of large game animals such as mammoth and reindeer which moved between Alaska and Siberia. From Alaska they later spread down the continent of America. They were adapted to hunting large animals in the Arctic, but soon adjusted to an amazing variety of climates, habitats and ecosystems. Descendants of the Siberians settled the thick forests of the Eastern United States, the swamps of the Mississippi Delta, the deserts of Mexico, the steaming jungles of Central America, and the open Pampas of Argentina within about just 2,000 years. By 10,000 BC, sapiens already inhabited the most Southern point of America, in Southern Argentina and Chile of today.

This spread of humans across America, testifies to the incomparable ingenuity and unsurpassed adaptability of our species of Homo sapiens. No other animal had ever moved into such a huge variety of radically different habitats so quickly without undergoing any significant genetic mutation and evolution. There are types of animals which you can find in all of these various habitats, like ants, but they are not the same ants. Ants in the deserts of Mexico are different from ants in the delta of the Mississippi, or from the ants in the Pampas of Argentina. It took millions of years for ants to conquer all these different places and get adapted to them. Sapiens did it in 1,000 to 2,000 years without undergoing any significant genetic mutations and evolution.

American animals (credit – Wikimedia)

The American blitzkrieg not only testified to the ingenuity of Homo sapiens, but also left behind a very long trail of victims. The American animals 60,000 years ago were much more varied than today. 60,000 years ago you would have found a  much more varied kingdom of animals than you find in America today. When the first humans marched down from Alaska to the plains of Canada and the Western United States they encountered mammoths, mastodons (a big species of elephant), rodents the size of bears, herds of horses and camels grazed in the Great Plains. There were over-sized lions and dozens of other large species which are very different from anything we know today in America or anywhere else in the world.

North America had fearsome saber-tooth tigers, with very long, sharp teeth as well as many species of giant ground sloth that weighed up to eight tons and reached heights of up to six meters. South America hosted an even more exotic collection of large mammals, reptiles and birds. Within a few thousand years after the sapiens arrived, most of these unique big creatures disappeared. According to current estimates, within that short interval of 1,000 to 2,000 years North America lost about 34 out of 47 genera of large mammals and South America lost 50 out of 60 genera of large animals. We are speaking here about genera, not about species. Each genera contained several species and they all disappeared. The genus of the giant ground sloth contained many different species and they all disappeared in about 2,000 or 3,000 years from the arrival of sapiens. The saber-tooth cats, for example, saber-tooth tigers that flourished in America for close to 30 million years disappeared within 1,000 to 2,000 years. The giant ground sloth, over-sized lions, Native American horses, Native American camels, giant rodents; all the members of the American elephant family and many more big creatures went extinct.

Smilodon (Credit – Wikimedia)

Thousands of smaller species, of smaller mammals, reptiles, birds, even insects and parasites disappeared, went extinct around the same time. For example, when the mammoth died out, all species of mammoth’s parasites, like mammoth fleas and mammoth ticks, followed the mammoth to oblivion. If we combine the mass extinctions in Australia and America, and add the smaller scale extinctions that took place as Homo sapiens spread from East Africa to other parts of Africa, Europe, and Asia, we can conclude that this first wave of sapiens colonization of the world, was one of the biggest and swiftest ecological disasters ever to befall the animal kingdom. Hardest hit of all were the large terrestrial mammals. The creatures which were closest to Homo sapiens, or most similar to Homo sapiens which were also terrestrial large mammals suffered the hardest hit. The more similar they were to us  the less chance they had of surviving.

So it continued.

(Credit – geography.howstuffworks.com/)

At the time of the cognitive revolution planet Earth was home to about 200 genera of large terrestrial mammals, mammals weighing more than 50 kilograms. At the time of the agriculture revolution, only about 100 genera survived. This means that Homo sapiens drove to extinction about half of the planet’s big terrestrial mammals long before we invented the wheel, writing, or iron tools, not to mention modern industry and modern weapons. This ecological tragedy, the extinction of many, many different species in a short time, was re-staged in miniature countless times after the agricultural revolution. The extinctions did not stop when sapiens became peasants and herders. The archaeological records of island after island tell the same sad story in the years during and after the agricultural revolution.

There were no more new continents that sapiens could discover and settle, except for Antarctica, but there were still many islands, which hunter-gatherers never managed to reach, like Madagascar. They were settled for the first time by farmers, peasants, and shepherds. Whenever these farmers reached a new island, the same tragedy happened as in Australia and America. It was on a smaller scale simply because it was a small island. An island would contain a rich and varied population of large animals but without any trace of humans. We have evidence for the appearance of sapiens in the archaeological record. A human bone or a spear point or piece of pot, pottery shows that Sapiens appeared and shortly afterwards most of the large animals and many of the smaller animals were all gone.

Evidence of destruction

Elephant Bird (Credit – Wikipedia)

One place when we can see this kind of tragedy happening is on the large island of Madagascar, about 400 kilometres east of the African mainland. Madagascar was remote from the African mainland so it was disconnected from what was happening in Africa. For millions of years the evolutionary processes on Madagascar was isolated from the rest of the world and produced a very unique collection of animals. Most notable among them was the elephant bird. The largest bird ever perhaps, it was a flightless bird, about three meters tall and it weighed about 500 kilograms, half a ton. It was the largest bird ever in the world. Also notable were the giant lemurs which were the globe’s largest primates ever. The lemur was a kind of primate. The biggest of them were larger than gorillas. These giant lemurs and the elephant birds and many, many other unique animals that existed on Madagascar for millions of years, suddenly vanished about 1,500 years ago, around 500 AD, precisely when we have the first evidence for humans arriving there.

Similar ecological disasters occurred on almost every one of the thousands of islands that you can find in the Pacific Ocean, in the Atlantic Ocean, in the Indian Ocean, in the Mediterranean, even in the Arctic Ocean. We have these island tragedies repeating themselves one after the other. Archaeologists have discovered on even the tiniest island, evidence for the existence of birds, insects and snails that lived there for countless generations, millions of years, only to vanish when the first human set foot on that island.

The third wave

Dinosaurs (Credit- the telegraph)

The first wave of extinctions accompanied the spread of the hunter gatherers, tens of thousands of years ago. The second wave of extinction accompanied the spread of farmers to islands like Madagascar. These two waves of extinction give us an important perspective about the third wave extinction which is happening today due to the spread of industry around the world. It is a very important and not so nice fact to know, that we Homo sapiens, never really manage to live in harmony with nature. Some people imagine that this clash between sapiens and the ecological system, these events of mass extinction, are a unique modern phenomenon, the result of modern industry and the capitalist system and things that happened in the last two or 300 years. This is not the case. Even if we go back thousands of years to the ancient agricultural societies, and tens of thousands of years to ancient forager societies, even then we have a lot of evidence for the way that sapiens simply destroy everything that stands in their path, and causes unique ecological disasters of the kind that no other species has ever caused in 4 billion years of evolution on Earth. There were previous cases of mass extinction of animals. The most famous happened 65 million years ago, when an asteroid from outer space hit Planet Earth, and caused the extinction of the dinosaurs, and many other animals. It never happened before, as far as we know, that a single animal species was responsible by its action for the extinction of dozens or hundreds or thousands of other species. This is what our species, Homo sapiens, managed to do thousands and tens of thousands of years ago.

(Credit – Wikimedia)

Perhaps, if more people were aware of the first and the second waves of extinctions, they would be less nonchalant about the third wave which we today are a part of. If we knew how many species we’ve already eradicated from the face of the Earth, perhaps we might be more careful, we might be more motivated to protect those species that somehow managed to survive. This is especially relevant to the large animals of the oceans. Unlike the large animals on land those of the seas and oceans suffered relatively little from the cognitive and agricultural revolutions. Until about three to five hundred years ago they were doing quite well. Over the last 300 years, they’ve been suffering more and more from the results of the new technology and new abilities of Homo sapiens. Many of them are today on the brink of extinction as a result of industrial pollution and from human over use of the oceanic resources. If things continue at the present pace, it is quite likely that whales, sharks, tuna, dolphins, and large sea turtles, will all follow the diprotodons, and the giant kangaroos, and the saber-tooth tigers to an early grave.

The Human Flood

(Credit – advocacy.britannica.com)

From all the large creatures of planet Earth, it is likely that the only survivors of the human flood will be humans themselves, and the farmyard animals that served as galley slaves in Noah’s Ark. You probably know the biblical story of Noah and his ark that tells us of a flood and how Noah went into the ark with his family, but also took representatives of all the animals in the world in order to save them from being driven to extinction by the flood. The true story, which you don’t find in the Bible, is very different. The true story is that the flood is us Sapiens, the flood of humans coming from East Africa and flooding over the entire world and driving so many animals into extinction. The only animals that we take into our ark to save  are not the representative of all types of animals, Kangaroos, diprotodons, whales and sharks and so forth. We take into the ark only a select few, the pigs, horses, chickens, cattle, and sheep, which we need for our own use. We take those that serve as our food, clothing, pets and working animals. All the rest are basically being, exterminated by the human flood.

If you enjoy reading my notes consider making a small donation to one of these charities. No donation is too small, you could change a life.


  1. serge says:

    Good job, Louise…


  2. Thank you! I enjoy the professor’s lectures and your lecture notes are very helpful.

  3. Richard says:

    Thanks Louise – brilliant work

  4. Mimzz says:

    Louise, I am doing the same course as you are and think that you consolidate everything beautifully! This is so helpful 🙂

  5. andrea ball says:

    I am certainly going to donate to the ‘Room to Read’ charity as a ‘thank you’ for these luscious notes. They really help me relax and enjoy the professor, knowing you will be writing down the tricky spelling of words like ‘diprotodon’ … You are an educator!

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