Friday, July 21, 2017

The 10,000 Year Explosion, Chapter 2: A Summary

Chapter 2 of Gregory Cochran and Henry Harpending’s The 10,000 Year Explosion: How Civilization Accelerated Human Evolution (2009) is called “The Neanderthal Within,” and examines the possibility that early humans outside of Africa interbred with Neanderthals, and how this affected human evolution.

Neanderthals and early Homo sapiens outside of Africa encountered one another and competed for resources – and early humans won out in about 10,000 years, perhaps because:
(1) our ancestors had projectile weapons (e.g., throwing spears, darts, bows and arrows) which they could use much more effectively with lighter, less bulky bodies (and so they required less calories to live);

(2) early humans were more intelligent;

(3) early humans had superior language abilities, and had better social organisation (Cochran and Harpending 2009: 25–26);

(4) and some suggest that early humans might have spread some bacteria or parasites which they had immunity against, but against which Neanderthals had not evolved immunity (Cochran and Harpending 2009: 28).
These are suggested explanations, though some may be wrong.

In any case, humans replaced Neanderthals and a revolution in material culture happened in the Upper Paleolithic in certain places like Europe where from 30,000 to 40,000 years ago humans invented all sorts of new tools and weapons, textiles, cave paintings, sculpture, and jewellery, as well as engaging in more long-distance trade (Cochran and Harpending 2009: 30). In Europe, this was called the Aurignacian culture (from c. 41,000–c. 26,000 BC), as well as the earlier Châtelperronian culture in central and south-western France and northern Spain (c. 43,000–c. 38,000 BC).

Such a revolution in material culture required a new propensity for invention, innovation and intelligence that was mysteriously absent in earlier periods of human evolution. Why did this happen?

Cochran and Harpending propose that this came about by the interbreeding of humans with Neanderthals in Europe and parts of Asia (Cochran and Harpending 2009: 36), though it did not happen in Africa. They propose that Neanderthals gave to early humans certain alleles (gene variants) which became common and conferred not only the ability to tolerate the cold or resist local diseases (Cochran and Harpending 2009: 54), but also new and advantageous cognitive abilities (Cochran and Harpending 2009: 56–57). They suggest that the gene microcephalin (MCPH1) that regulates brain size and the FOXP2 allele that has a role in speech and language abilities might have been acquired from Neanderthals (Cochran and Harpending 2009: 62–63).

Neanderthals had evolved bigger brains, and this may well have meant a greater level of cognitive ability and intelligence in certain ways, which were useful in hunting big game and in the colder, harsher environment of Europe (Cochran and Harpending 2009: 55).

So Cochran and Harpending argue that after c. 50,000 years ago early humans outside Africa – especially in Europe – acquired new, useful and greater cognitive abilities from admixture with Neanderthals, which was a genetic precondition for the cultural revolution of the Upper Paleolithic peoples (Cochran and Harpending 2009: 64).

This thesis is one of the more controversial aspects of The 10,000 Year Explosion, and other scientists argue that, while interbreeding did occur, it was rare and biologically and genetically unimportant (Cochran and Harpending 2009: 40).

It appears that, while early humans in Eurasia did interbred with the Neanderthals (and Neanderthals had in turn evolved from Homo erectus populations) (see here), the Neanderthal genetic contribution to modern Europeans is low: some put it as low as 1.5–2.1% (Prüfer et al. 2014). (For a useful family tree, see here). By contrast, Lohse and Frantz (2014) found that Neanderthal admixture occurring in ancient Eurasia was at a higher rate of 3.4−7.3%.

This remains a controversial issue, and here is some recent evidence in the videos below:

Also, some speculation on Neanderthal intelligence:

At any rate, because early humans developed better hunting techniques, required less food than Neanderthals, and had a more varied diet – that is, because they were more successful in evolutionary terms and better adapted to their environment – this led to greater population density than in previous human societies (Cochran and Harpending 2009: 33), and greater population density in turn allowed more mutations and more random creation of better traits by sexual reproduction in early human populations on which selection and evolution could work.

Finally, here is a chronology of events in prehistory relevant to the issues in Chapter 2 of The 10,000 Year Explosion:
300,000–250,000 – Homo heidelbergensis evolves into Neanderthals outside Africa

c. 158,000–38,000 BC – the Mousterian (or Mode III) culture or archaeological industry, of flint tools mainly associated with the Neanderthals, and some early humans, in Eurasia

125,000 years ago – Homo sapiens reached the Near East, but evidence suggests they retreated back to Africa, as their settlements were replaced by Neanderthals

108,000–9,700 BC – last Ice Age

c. 73,000 BC (± 900 years) – Lake Toba supervolcanic eruption (in Sumatra, Indonesia). This is the largest known explosive eruption on Earth in the last 25 million years. According to the Toba catastrophe theory, it had global consequences for human populations: it killed most humans living at that time and is believed to have created a population bottleneck in central east Africa and India, which affects the genetic make-up of the human world-wide population to the present

75,000 years ago – Homo sapiens left Africa again about across the Bab el Mandib, connecting Ethiopia and Yemen into Middle East

60,000–50,000 BC – outside Africa, Homo sapiens lives in Near East, Greece, south Asia, New Guinea and Australia

c. 58,000 BC – most areas north of the tropics not inhabited by Homo sapiens because of the cold and difficulty of food supply

c. 50,000–40,000 years ago – southeast Asians reach Australia; in Australia by 46,000 years ago at the latest

c. 43,000–41,000 BC – Cro-Magnon Homo sapiens reached Europe from the Near East, eventually replacing the Neanderthal population by 40,000 years ago

c. 43,000–c. 38,000 BC – the Châtelperronian culture in central and south-western France and northern Spain

c. 41,000–c. 26,000 BC – the Aurignacian culture is found in Europe (probably associated with GoyetQ116 type people), the archaeological culture of the Upper Palaeolithic; this first appears in Eastern Europe around c. 41,000 BC, and spread into Western Europe c. 38,000 and 34,000 BC, but replaced by the Gravettian culture c. 26,000 to 24,000 BC

39,000–37,000 BC – Neanderthals die out in Europe

35,000–12,000 BC – European hunter-gatherers descend from a single ancestral population with no significant genetic inflow from other regions

c. 29,000–c. 22,000 BC – the Gravettian tool-making culture of the European Upper Paleolithic of Vestonice cluster type people; ice age glaciation seems to have wiped out Gravettian culture people c. 22,000 BC

28,000 BC – East Asia was reached by Homo sapiens

28,000–13,000 BC – last cool phase of the Ice age; humans withdraw from north Eurasia to more southerly areas

c. 27,000–18,000 BC – Last Glacial Maximum (when the ice sheets were at their greatest extension) c. 24,500 BC; deglaciation began in the Northern Hemisphere gradually from c. 18,000 to 17,000 BC

26,000 BC – last group of Neanderthals disappear from southern Spain
The blog of Gregory Cochran and Henry Harpending:
West Hunter
Cochran, Gregory and Henry Harpending. 2009. The 10,000 Year Explosion: How Civilization Accelerated Human Evolution. Basic Books, New York.

Lohse, Konrad and Laurent A. F. Frantz. 2014. “Neandertal Admixture in Eurasia Confirmed by Maximum-Likelihood Analysis of Three Genomes,” Genetics 196.4: 1241–1251.

Prüfer, K. et al. 2014. “The Complete Genome Sequence of a Neanderthal from the Altai Mountains,” Nature 505.7481: 43–49.

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