Out of Africa 2, Evolution of Homo Sapiens

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Are Neanderthals and Anatomically Modern Humans the same species?

Before talking about the origin of Homo Sapiens, we need to consider (just concentrating on the major groups) whether to include  Neanderthals and their close cousins, the Denisovans were Homo Sapiens.

Neanderthal woman, Bacon Cph 

Neanderthals evolved in Europe and ranged across southern Europe, the Middle East and Asia as far as the mountains at the western edge of Mongolia. They survived until about 40,000 YA. 

The closely related Denisovans (from ‘the cave of Dennis’ in Siberia) were a little more elusive. They ranged mainly across Asia, surviving in New Guinea perhaps as late as 11,500 YA.

 In Western Africa, there were pockets of archaic humans that survived through till at least 13,000 years ago.  

(Modern) Human DNA, in Eurasia and New Guinea,  contains DNA from Neanderthals and Denisovans, indicating ‘AMHs’ (Anatomically Modern Humans) bred with them. 

Modern African DNA indicates interbreeding with an, as yet unidentified, archaic human.

Archeologists have found bone fragments of hybrids: an AMH/ Neanderthal hybrid (in Italy) and a Denisovan/ Neanderthal hybrid (in the above mentioned Siberian cave), both with Neanderthal mothers.  There is a more complete specimen from Romania of a 15-16 year old male with mixed modern and archaic anatomical features dated as recently as 30,000 YA ago.

If AMHs, Neanderthals and Denisovans interbred (and the consensus is that they did) shouldn't they be (by definition) part of the same species?  

This is the reason I have stuck with the somewhat clumsy term ‘Anatomically modern Humans’ (‘AMHs’) rather than use  'Homo Sapiens'  for AMH which assumes this issue is already decided.

While I have have some sympathy for suggesting this is not fully settled and we should still leave open the possibility of inviting Neanderthals to join our 'club' ( call them 'Homo Sapiens Neanderthalis') this is not the consensus, most see them as a separate species to 'us'. 

(In explaining this, I will concentrate on the more populous Neanderthals, though Denisovans genes were more successful than the Neanderthal ones). 

Being able to interbreed may not be enough to be the same species even though it is the usual test of separateness or not. 

Firstly, some (gene) studies have suggested successful interbreeding with Neanderthals might have been uncommon. 

If this were true, and even if it was just from lack of physical attraction, the further we get from successful interbreeding, the closer we get to them being separate species. 

Evidence for Y chromosomes and mitochondrial DNA

There is no Neanderthal DNA in modern human mitochondria and not a lot in Y chromosomes. 

This has led some anthropologists to variously suggest that Neanderthal/AMH hybrids were infertile, aborted, or sickly. Again, if this is true, the further away from viable, fertile offspring we get, the closer we are to species separation (‘speciation’).

Confusingly, the mitochondrial DNA and Y chromosome stories point in different directions.

Mitochondria are small factories in cells that produce energy and protect the cell from oxidative enzymes. They have their own DNA, which mutates more rapidly than nuclear DNA, and all mitochondria and mitochondrial DNA comes from the mother.

Neanderthal m-DNA is very distinctive. If there is  none in modern humans, it suggests that a hybrid with a Neanderthal mother is less viable or might be infertile, and not the cause of our Neanderthal genes.

Moving back to the DNA found in chromosomes : women have two X chromosomes, men have an X and a Y.  It is men , through their sperm, that determine the sex of the child.

While there is a mechanism for a pair of chromosomes to sometimes swap matching parts of themselves, sexual reproduction is the main way advanced forms of life have of mixing genes. (Swapping genes can happen between X and Y but not in the sex-linked portion).

So if there is less Neanderthal DNA in the Y chromosome, maybe it is the boys with a Neanderthal father that could have been infertile or have low survival.  

Other reasons for these findings

All these possibilities are not necessarily mutually exclusive and there are also are other possible factors that might have contributed to these genetic findings.

The above may be due to sampling errors. Gene technology is very new (the initial draft for the  Neanderthal genome was in May 2010). We have few Neanderthals to test and even fewer hybrids and it is the hybrids which will contain the answer. 

Now, if  we can identify more hybrids, even from bone fragments, and investigate them, perhaps we can see what really happened.

As mentioned, Neanderthals had fewer numbers than European AMHs (Cro-Magnons). They many have left a smaller genetic footprint from simple dilution.

And yet, there is a darker, Darwinian, possibility for our poor Neanderthals and their genes. Unfortunately it does seem to be at least part of the story.  

Neanderthals were a physically robust group and physically well adapted to hostile, changeable climate in the Glacial period. Still, they mostly only lived till 35 whereas Cro-Magnon adults often lived till 50. 

Several studies of Eurasian fossils suggest initial levels of Neanderthal DNA were about 3 times what is seen today and a lot of the surviving Neanderthal DNA is 'non-coding' (so called 'junk' DNA). It seems natural selection was particularly brutal towards Neanderthal DNA, and it acted over a relatively short period of time.

Some paleo-geneticists suggest that Neanderthal populations were more in-bred, with harmful or weak genes. Whether this was true or not, there is no doubt Neanderthals were seriously outmatched by AMHs.

They had simpler tools with little adaptation over time and simpler social organisation and clothes whereas AMHs literally specialised in innovation. 

AMHs made throwing spears 279,000 YA and 130,000 Y.A.. Even before leaving Africa they developed art, pigments, glue, decorations, beads, and multiple tools and traded (obsidian) over 300 KM distances while Neanderthal trade networks were much shorter

By 60-70,000 YA. AMHs had developed hafted tools and bone and stone arrowheads ( 60-70,000 YA).

When AMHs finally left Africa, they were in small numbers (see below) and yet the Neanderthal phenotype lost really badly and fairly quickly, only overlapping Cro-Magnon man by 5,400-12,000 years. 

I won't document all the theories for extinction of Neanderthals, but some suspicion has to fall on the arrival of AMHs, whether they simply 'out competed them' or brought new diseases. At least, there is less support for early theories that they hunted them.  

Chrono-species

Another factor that might explain the apparent interbreeding of two 'species' would be the concept of 'chrono-species'. 

All (or most) of the species of Homo are ‘chrono-species’ (as discussed in last month's blog). 

Take the Chihuahua and the Great Dane. They are sufficiently different that they cannot breed (even if they wanted to). They would be separate species, except that there is a chain of dogs in the middle that allow them to share the same gene pool.

In ‘chrono-species’ there is increasing separation over time and the intermediate links die out, leaving two or more  separate species. When this separation occurs is difficult to say, and if we go back in time to when the species have not fully separated, (for example to advanced versions of H. Erectus and early H. Sapiens), we might very well get interbreeding, though we would consider the two as different species if we are viewing them from a latter point in time.

(On the same vein and while we don’t think this is what happened, if AMHs and Neanderthals both bred with an archaic link and not each other, this would allow an exchange of genes while they are separate species).

Back to the question, where did H. Sapiens evolve?

H. Erectus, coming from Africa, colonised a large part of Eurasia starting about 2 million years ago, though in nothing like the numbers of AMHs. 

Where did AMHs evolve?

The earliest specimen of AMHs were found in North Africa  (Jebel Irhoud, Morocco about 300,000 years ago).

At first there was a mix of ‘modern’ and ‘archaic’ features. The modern skull features are a straight high forehead with a box-like head , smaller teeth and a prominent chin. The more ‘archaic features’ involve a thicker skull, the low, receding forehead, a somewhat oval skull, distinct brow-ridges and prognathism.

Over time, AMHs approached something more modern ( e.g. ‘Herto man’, Ethiopia 160,000 years ago progressing to another specimen from Tanzania 120,000 years ago).

They also developed a new (middle) Stone Age tool technology called ‘Aterian’.

AMHs spread to Israel/ Palestine and the Arabian peninsula fairly early. 

These regions had much less desert at the time and shared a number of animals with Northern Africa. Their habitats being virtually connected.

The first specimen outside of Africa was a jaw bone fragment found near Mount Carmel in Israel,  dating back to about 190,000 YA. The characteristic Aterian tools have been found in Israel and the Arabian Peninsula also dating to about this time.

There is equivocal evidence (damaged fragments) of a surprisingly advanced form of AMHs in Greece 200,000 YA. 

Characteristic teeth and tools have also been found in China dated to 100,000 YA and modelling suggests AMH genes entering the Neanderthal gene pool around 100,000 YA.

While AMHs probably started in Africa, they would continue to evolve (and swap advantageous genetic adaptations) over the whole of whatever territory they had spread to.  

If it is confirmed that a truly advanced form made it to Greece so long ago, this might challenge the current narrative, but for the moment it seems that AMH first evolved in Northern Africa and spread out (with a mix of archaic and modern features) at least to the adjacent parts of the Middle East.

And then disaster struck (more on this in a minute).

3 models of H. Sapiens evolution

There are three main theories of how Homo Sapiens evolved from H. Erectus. 

The multiregional hypothesis, suggests that so called ‘major racial groups’ arose because H. Sapiens emerged form H. Erectus at multiple sites.

 Modern Australasians evolved from an Australasian H. Erectus, and so on. 

As first described, such a view is too extreme to fit the facts 

The ‘out of Africa’ theory suggests H. Sapiens evolved in Africa and then spread to Eurasia and might have been involved in pushing the other ‘archaic humans’ into extinction. 

The Assimilation model is a variation on this : AMHs evolved in Northern Africa and they partly assimilated the archaic humans they came into contact with.

A time of Disaster   

From around 350,000 YA (the period when the AMHs, Neanderthals and Denisovans were evolving) the climate was becoming somewhat unstable. It was beginning to alternate between warmer , wetter periods and cold, very dry, periods. Still, up to 200,000 YA. was a good time for the early AMHs. 

The world had been cooling for tens of millions of years but for that moment was going through a wetter, warmer phase and the Sahara was green. .

Then, around 115,000 YA, the climate turned murderous.

A Glacial Period began in Northern Eurasia (and North America) resulting in shrinking habitats and it lasted through till 11,700 YA.

Worse than that, the cold diverted rains away from Northern Africa and the Middle East triggering mega-droughts that went on and on and on, with massive desert expansion .

But something worse, far worse, was just about to happen, a great cataclysm. Around 75,000 YA  Mount Toba in Sumatra  erupted. 

The eruption was 12 times more explosive than the largest volcanic eruption in recent history (the 1815 eruption of Mount Tambora in Indonesia, which caused the 1816 ‘Year without Summer’ in Europe).

Some say it had little impact on the environment, others say it caused a ‘nuclear winter’ (if one can still use a somewhat debated concept). Whether this was the cause or not, this time saw a massive depopulation event, involving several mammalian species.

The groups of AMHs outside of Africa are thought to have died out, any in Europe were later replaced by Neanderthals and back in Africa, they were pushed to the brink of extinction.

Genetic models point to a serious ‘population bottle neck’ in those AMHs that survived in Africa (a sudden reduction in genetic diversity).

It was almost the end for our Anatomically Modern Humans.

Today's humans are said to be descended from a very small population,  between 1,000 and 10,000 breeding pairs (70,000 YA) and the common female ancestor of all living humans on earth (mitochondrial Eve) may have lived as recently as 150,000 YA.

Recent ‘out of Africa’

We now believe that a very small group of AMHs left Africa (this time successfully) 70,000–50,000 years ago and mainly followed the Southern Route to the Arabian Peninsula (sea levels were lower then but water transport was also available to them). 

They continued to spread relatively rapidly along the coast of Asia to reach Australia around 65,000–50,000 years ago (see red arrows in map).

An early offshoot followed a northern route via the Levant to West Asia, finally reaching Europe around 48,000 YA..

Behavioural Modernity

There are subtle physical differences between Cro-Magnon (early European AMH's) and the humans of today, showing we have been continuing to evolve, even over recent times, but the intellectual difference between Neanderthal and Cro-Magnon Man (40,000- 10,000 YA) was stunning.

Cro-Magnon, Charles Robert Knight,1920

While Neanderthal was still an advanced form of homo, Cro-Magnon had a rich culture with sophisticated stone working, advanced symbolism and cave paintings that stood in stark contrast to our poorer cousins.

It is described as 'behavioural modernity'. Some saw it

as an adaptation to the harsh environment in Europe at the  time but, as mentioned, it was already present in 

considerable measure and gaining momentum before AMHs left Africa. 

It is an area in which (we) AMHs really excelled, 

especially innovation, and it led to our success.

It goes without saying that Homo continues to evolve slowly and subtly, with improved adaptation to the environment in which we find ourselves and sharing these genetic adaptations across a gene pool now in the billions. 

Lately, this has been greatly overshadowed by social evolution (technological and social revolutions), where we are literally creating these new environments. The amount of change has been extraordinary and the pace is getting faster all the time, even in living memory.

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Out of Africa 1, the Evolution of Man

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Primates evolved 55 million years ago (MYA) ago in a world very different to our own.

Crab-eating_Macaques, Bernard Dupont, 

10 million years before, there had been a massive asteroid strike (or strikes) causing a 'mass extinction event' including most non avian dinosaurs, initiating a golden age for mammals.

The main land masses were closer than today. Eurasia and America were just separating; Australia and Antarctica were much closer and India was still a large island south of Eurasia.

It was very hot and steamy, in no small part due to greenhouse gases (later sequestrated into our fossil fuels and lying frozen in Siberian peat bogs). Equatorial forests stretched all the way to Antarctica.
It was an ideal world for the evolution and spread of a teeming population of tree dwellers and small mammals.

But it was also, for several reasons,  a climate maximum, heralding an inexorable cooling over many millions of years leading up to the Last Glacial Maximum about 19,000 BC.
Over this long period, tropical forests eventually shrank to the equatorial regions, being replaced by deciduous forests, grass lands and deserts. Permanent ice eventually formed at the poles. (Non human) primates eventually retreated from Europe, leaving only the Barbary apes of Gibraltar.

Current Primate range Napier and Napier, Wiki

This change was slow and 25 MYA there was still plenty of space for the Great Apes to evolve and flourish in the equatorial forests, through till about 15 MYA when the shrinkage of their habitat really began to 'bite'.

From our tree-dwelling forebears mankind inherited  forward-looking eyes, acute binocular vision (more often seen in predators) and grasping hands .
While there are many interesting suggestions, there is no fully accepted explanation of why our forebears moved to an erect posture and a bipedal (striding) gait, as opposed to what other apes and monkeys now do when they walk. Maybe it was a combination of things.
There is, however, a prelude in how apes use their upper limbs for grasping, manipulating (feeding and grooming) and their rear limbs for walking along branches. Apes also they tend to sit erect when feeding or resting. The potential for an erect posture and bipedal gait  was already there and that was the way 'we' 'decided' to go, and it worked .

Australopithecus.

Most Apes and monkeys remained in the shrinking jungles, but some monkeys (e.g. baboons), and some apes (e.g. chimps and Australopithecus) moved into the savanna (mixed grass/woodland) that was spreading out over East and Northern Africa.

To use a very archaic term, the Genus Australopithecus could be described as a ‘missing link’ between Apes and Homo (having features of both). They evolved around 4.2 million years ago, becoming widespread, eventually dying out about 1.9 million years ago.

They were the first of our ancestors to walk erect on two legs, sleeping in the security of trees at night. They had brains roughly 35% of the size of a modern human brain. Most were slender and covered in fur. The larger of the Genus reached 1.4 meters, many were smaller and females were distinctly smaller than males.

The name 'Australopithecus' comes from a combination of Greek and Latin, meaning ‘Southern Ape’ (from a full specimen discovered in South Africa).

They lived on fruit, vegetables, and tubers, and perhaps easy-to-catch animals such as small lizards.
The ‘Genus’ Australopithecus is defined as a group of ape species and subspecies that had features between apes and ‘homo’. In other words they walked erect.
We have no way to be sure if they are all as closely related as we think, we have lumped them together on similarities, often on the basis of fragmentary evidence. Some anthropologists variously propose lumping them with homo, splitting off some of them into other existing genera or using additional suggested genera to subdivide them. see here
This problem of classifying their finds is one of the great challenges for archaeologists and anthropologists who use morphology, habitat and behaviour as a guide to species, subspecies and genera (more on this later).

Cutting ties 

Genus Homo diverted from the Genus Pan around 5 million years ago, as measured by differences in DNA. Genus Pan is our closest relatives amongst the apes, chimpanzees being their only surviving representatives. 

In Genus Homo two chromosome pairs fused to reduce our chromosomes to 23 pairs instead of the 24 pairs in other apes, decisively separating Homo from all other surviving apes.

We currently need living, dividing, cells to count chromosomes, so we don’t know when this reduction happened, but it is most likely early. We also don’t know if Australopithecus had this too, but it is definitely possible.

Genera, Species and Subspecies

The concept of a species seems simple enough: (the largest grouping of organisms that can crossbreed with one another and produce fertile offspring).  A ‘genus’ is a group of related species with a common ancestor, and subspecies are groups within a species that look different, sometimes significantly so, but can still readily crossbreed if given the opportunity.
'Polytypic' can refer to a Genus (with lots of species) but more often refers to a species with lots of subspecies. The main species of homo over time were polytypic.

‘Breeds’ of domesticated species (like the multiple types of dogs, horses etc) are a special example of ‘subspecies’. If dogs were somehow long extinct and only a few partial remains were available, an archaeologist would likely think he had discovered several different species. In fact domesticated dogs, some wild dogs, and various wolves are all (currently) recognised as one species!

Early man cannot compete with the sort of diversity of Canis Lupus Familiaris (domestic dogs) has/have. The evidence is still fragmentary, especially in the distant past, but over time we have begun to believe that interbreeding of different-looking contemporary forms of Homo was far more common than we ever dreamt could be possible, and this explains some of the combinations we have found.
It's hard to be absolutely sure, but much of what we assumed were different contemporary species were subspecies. 

Part of this new understanding followed advances in understanding of how evolution actually works, and part of  it followed the realisation that modern man interbred with Neanderthals and Denisovans as soon as he reached Eurasia from Africa (more on this later).

Evolution and subspecies.

Evolution occurs when there is variation between individuals (within a species) and some of these variations have an advantage for survival and reproduction, either dependent on a certain habitat or an over all advantage.

Sub-species occur in nature when groups of the same species become separated over time or place and they then adapt to different environments or simply experience genetic drift. 

With several subspecies of early man (capable of interbreeding). Each had particular mutations that might be an advantage 'over all' or in certain habitats. Interbreeding, ensured that any differences that might prove advantageous could be combined and not lost forever over time.
Specific combinations may work well together. Such a wider variation allows penetration into niche habitats and faster adaptation to habitat change so was often a survival advantage for the species.

Chronospecies
This interbreeding and evolution within subspecies leads to the concept of 'chronospecies'. A chronospecies is a species which slowly changes over time until it cannot be considered to be the same species as the originating genetic line (had they existed at the same point in time). 

Chrono-species Ian Alexander, Wiki

It might be just genetic drift and/or different environments causing such an accumulation of differences that eventually interbreeding becomes unlikely (e.g. loss of sexual attraction, mating rituals or physical differences).

While natural selection can favour  interbreeding to foster a larger gene pool with greater diversity, it can also favour separation of subspecies into separate species ('speciation') .
This happens when the 'in-between' or older varieties become at such a disadvantage relative to the new variety that the benefits of the larger gene pool (to the species as a whole) is outweighed by the cost.
Such a thing occurs more rapidly when the habitat of the subspecies is very different and the adaptation needed for each niche habitat is different.

In the development of a chronospecies, each change is small and interbreeding occurs along the 'chain'. The point at which we would say a new species has 'arisen' is arbitrary. The evolution of man followed just such a pattern.

While there is considerable uncertainty and debate, the minimalist view (currently favoured) leaves Homo Habilis, Homo erectus and Homo sapiens (and their respective contemporaries) as the only three species of Homo.  

Encyclopaedia Britannica  see here

Gone are the multiple ‘species’ that were proclaimed every time an old time archaeologist found remains that varied from what had been found before.

We need to be cautious not to see evolution as always a march from primitive to more advanced and complex, though this can happen.
Natural selection doesn't care about human value judgements, it only cares about adaptation (survival) and reproduction. It just so happens that many of the adaptations leading to modern man had a huge survival advantage. 

Homo Habilis

Homo habilis was one of the earliest species of ‘man’ (homo), 2.4 to 1.4 MYA (overlapping Australopithecus by half a million years).
The arrival of Habilis marks the official start of Genus Homo.
‘Habilis’ is Latin, the same root as ‘able’ and has a number of related meanings like ‘skilful’. ‘Homo Habilis’ has been translated as ‘handy man’, tool maker.

Female H. Habilis, Élisabeth Daynès 

We initially used evidence of fashioning of tools as the  defining feature of Genus Homo, but advanced types of Australopithecus have been shown to fashion primitive tools maybe even a million years before Homo arose.

We don’t know for sure if Homo Habilis is a direct ancestor of modern man as we first believed, and we don't know 'his' relationship with a number of other early forms of Homo (like H. rudolfensis, another contender for the title of being our ancestor. We don't know whether they were separate species or subspecies).

The oldest Homo remains, LD 350-1, (as yet unclassified into a sub species) is from Ethiopia 2.8–2.75 million years ago (mya) confirming that 'homo' evolved in the African savanna.

Homo spread to Asia about 2 million years ago. There were manufactured tools from 2.1 MYA found in Shangchen, China and specimens found in the nation of Georgia 1.85–1.77 MYA. These were thought to be from Homo erectus but the Georgian specimens and brain size are decidedly primitive, so it might have been a variety of Homo Habilis or maybe something in between.

Homo Erectus

The award for the first species of man that spread beyond Africa usually goes to Homo Erectus.
H. Erectus lived from between 2 MYA to as recent as 110,000 years ago, overlapping H. Habilis by almost half a million years and Homo Sapiens by about 200,000 years. Being a Chronospecies the later versions of H.Erectus would be more advanced.

H. Erectus dispersal, Sheila Mishra Here

It is the longest surviving of all human species, being present nine times as long as our own species, Homo sapiens.

H. Erectus was highly successful, and mobile, and spread widely over Africa and Eurasia. 

It was H. Erectus that first began to hunt medium-large animals in groups (such as bovines or elephants), moving to a diet richer in meat. Erectus would have had some tribes that routinely followed migrating herds as well as moving into other niche habitats.

Erectus tamed fire a million years ago , and maybe earlier, and he made water craft probably for fishing, but was able to colonise at least a few islands (eg Indonesia)

With longer legs and relatively shorter arms (no longer climbing trees) loss of hair, and adaptation in sweat glands, he was able to run longer distances in the heat . H. Erectus became bigger, faster and smarter (with an average brain capacity at least a third bigger than H. Habilis).


There is a conflict in the best pelvis for running on two feet and the ability to deliver babies with large heads.
In 'Modern man ' children are dependant on their parents for longer (with the continued move to more monogamous bonding in parents) allowing considerable brain growth after birth.
Larger brains require a lot of energy and are best supported in a hunter-gatherer society by cooking food and using more meat in the diet, requiring co-operative hunting of large animals, mostly by males working together.

H. Erectus is a chronospecies with a very long history and a very wide range. He evolved over time and there were many subspecies. The larger subspecies had body proportions similar to modern man, with distinctly larger brains (up to 60% of H. Sapiens). This is in sharp contrast to the smaller and earlier subspecies (or species) of Erectus.

The main African versions is often called ‘H. ergaster’.
Homo heidelbergensis (700,000 to 300,000 years ago in Africa and Europe) is the subspecies (or maybe chronospecies) closest to H. Sapiens.

The oldest H. Erectus remains are found in Kenya. The majority view is that H. Erectus also evolved in Africa and spread over Euro-Asia via the Middle East then on to Asia (including China and Indonesia) about 2 MYA. As said, he also made it to islands and must have made primitive water craft. There is some debate when H. Erectus arrived in Europe, whether it was soon after or somewhat delayed because ‘he’ preferred a warmer climate.

Next blog is the conclusion of  'out of Africa'

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The Missing Neolithic Farmers of Old Europe

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 In last month’s blog we discussed the dawn of the Neolithic Age. It is believed that the transition to  

Urfa Man Turkey 9,000 B.C.

Neolithic farming occurred in the Levant, most likely when a 1200 year drought brought a catastrophic end to the local Mesolithic Age. A few permanent settlements survived this catastrophe (like Tell Qaramel near modern day Aleppo) but the shortage of forage likely forced greater reliance on cultivation of 'wild' strains of plants and grains (and presumably primitive irrigation).

 At first ‘pre pottery Neolithic’ settlements (PPNA, 10,000–8,800 BC) were similar to the Natufian Mesolithic villages except for being larger and (much) more sophisticated (e.g. Jericho).

There were variations over time and distance but in the later ‘a-ceramic’ period (PPNB, 8,800–6,500 BC) Anatolia (Turkey) emerged as a Neolithic powerhouse, incorporating animal husbandry, 'domesticated' strains of grain and other crops and farming techniques some of which were discovered elsewhere (such as Mesopotamia and the Zagros region of western Iran).

It was Anatolian farmers and fisher folk that island hopped to reach Greece (around 7000 BC, before the official start of the pottery Neolithic Age in the Middle East, 6,400 BC). They established a large number of thriving maritime settlements  over the northern Mediterranean and eventually the Atlantic coast. They followed two land routes into Europe, eventually reaching Scandinavia and crossing the English Channel around 4,000 BC ( the map to the right and below is only to roughly show some of these routes).

Wiki

This was not the spread of ideas, this was large scale movement of farmers, their crops and animals. They intermarried with the nomadic and Mesolithic hunter-gatherers of Europe but they dominated them with their superior numbers.They cleared virgin forests, tilled the soil and domesticated local plants and animals. 

It was perhaps in central and south eastern Europe that saw their greatest achievements supported by a vast trade network. There they lived in heavily fortified settlements, like Sesklo in Greece which in 5000 BC may have housed  an incredible 5,000 people, one and half millennia before the invention of the wheel and only a few hundred years after the first (Sumerian) city (Eridu in Mesopotamia) was said to be established.

Archaeologists have found the oldest gold treasure in the world in Bulgaria (4,600 BC to 4,200 BC)  

Varna necropolis, Bulgaria Wiki

the earliest evidence of copper smelting in Serbia (5500-5000 BC, 1.5-2 millennia before the Copper Age) and the world's oldest salt mine in Hallstatt Austria (about 5,000 BC).

The 'Old European culture' varied over time and distance but their numbers and sophistication continued to grow with the arrival of the potter’s wheel and ox-drawn carts. A few archaeologists have even suggested some of their pottery symbols were a form of proto-writing hundreds of years before the Sumerian pictograms.

It was they that built many of the first European megaliths, long barrows and passage tombs. Some of the ‘mega settlements’ in central and South Western Europe around Western Ukraine (4000–3500 BC) may have housed 20,000 to 46,000 people.

Almendres Cromlech from 6,000 BC Portugal, Wiki

The first Neolithic farmers of old Europe reached their peak around 4,000 BC.
Around 3,500 BC their population began to collapse relatively abruptly. In many regions the depopulation was catastrophic. Britain  lost 90 % of its Neolithic population in three hundred years. In a few areas it was almost 100%, in other areas it was less and a few areas were relatively spared.

The Pelasgians, Minoans and Leleges (Greece), the Iberians and Basques, the Nuragic people of Sardinia, the Sicans and the Elymians of Sicily and the Etruscans are all suggested (with different levels of certainty) to be related to these older Europeans. See my earlier discussion of the first people of Greece here.

The artwork of these ‘old Europeans’ lead the celebrated archaeologist Marija Gimbutas to suggest they were ‘matristic’ and peaceful, worshiping the mother goddess and that they were overwhelmed by the war-like ‘paternal’ (fairer) Indo-Europeans pastoralists from the great Steppe, with their horses, weapons and male Gods.

Hamangia culture Bulgaria, wiki

There is no positive evidence for such a whole sale conflict and besides, the Indo-Europeans didn’t begin arriving in any numbers till 500 years after the start of the catastrophe, moving in to fill a relative vacuum.The Indo-Europeans weren't to finish their settlement of Europe till about 1,000 BC (towards the end of the European Bronze Age).

What happened to the missing Neolithic Farmers of old Europe?

History is replete with examples of sudden inexplicable collapse and disappearance of Neolithic cultures and civilisations, leaving only sadness and mystery behind. The usual ‘suspects’ warfare, climate change and soil exhaustion are not satisfactory here.

The old Europeans were sophisticated farmers who knew all about organic fertiliser and pasture rotation. The depopulation seemed to have hit different areas at slightly different times but a population ‘bust’ seemed much more likely after exponential population growth, as if it was punishing success.

Perhaps the population growth made food security more precarious and unable to respond to an external shocks. In some cases there was an increase in population nearby suggesting people were fleeing from something.
It is now suspected that the population density and unhygienic conditions led to an outbreak of disease, which spread from village to village over the trading routes. 

We already know just how devastating an epidemic can be in a population that has no natural immunity and the shift to Neolithic lifestyle made people much more vulnerable to epidemics.
There was already a long tradition 6500 BC -2000 BC in central and south-eastern Europe of villagers burning their whole village and moving every eighty years or so. Sometimes they would return to an old site much later and rebuild on the rubble. Was it denying another tribe use of the village while allowing the land to lay fallow? Was there religious or social reasons? We don’t really know, but it could have been related to vermin or disease.

If it was to cope with disease, burning their homes and fleeing to other areas no longer worked for the Neolithic farmers.

The scale of devastation with some areas massively depopulated (some less affected and a few relatively spared) is reminiscent of a later catastrophe, the Black Death and it seems that they were dealing with something of similar virulence. In a previous article we discussed the evidence emerging that they were indeed dealing with first pandemic of the Plague, with an early primitive strain but at least equally as devastating.here . 
Where did the plague initially come from?  We know its ancestor Yersinia pseudotuberculosis came from the far east so this early form of Plague likely came via the Great Eurasian Steppe.
It is pure speculation but possibly a more peaceful contact with Indo-European traders still proved fatal to the old people of Europe, long before the Indo-Europeans moved into Europe in any numbers.

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