Genetics and Human Evolution
Recent technological breakthroughs in genomic analysis, combined with archeological, paleoanthropological, linguistic and other information, now give us an unparalleled opportunity to trace humanity’s evolution and movement in time – how we developed, differentiated and interbred many times, and arrived at our present planet-wide population.
Until recently, the leading theory of human population descent, known as the “serial founder” model, envisioned modern humans expanding out of Africa and the Near East around 50,000 years ago and leaving descendent populations along their routes of migration. The settlements of these groups were thought to remain unmixed for tens of thousands of years, and consequently were classified according to location, superficial appearance and culture as East Asians, Caucasians, West Africans, Native Americans and Australasians.
We now know, thanks to studies of ancient DNA (aDNA), that the serial founder model is wrong. It turns out that present-day populations are actually mixtures of highly divergent populations that no longer exist. Nor are present-day populations, thanks to perennial migration and mixing, exclusive descendants of the populations that lived in the same locations ten thousand years ago.
The announcement of evolutionary trees based on variation in mitochondrial DNA in a 1987 edition of Nature, followed by a study on evolution of the human Y chromosome a few years later, staggered the world. From the pattern created by the random genetic variations on both of these lineages, geneticists can conclude facts rather than suppositions and can of course construct family trees for everyone alive today.
DNA molecules make up the human genome, the genetic code that each of us inherits from our parents. DNA consists of twin chains of molecules called nucleotides made from the chemicals adenine (A), cytosine (C), guanine (G) and thymine (T). Each chain, broken up into 23 chromosomes, adds up to about three billion chemical blocks in length. Genes are fragments of these chains, generally around a thousand nucleotides long, each one telling something about how the body is built.
Each time egg and sperm are created, the approximately 3 billion base pairs of DNA comprising our genes are copied. Random variations in these inherited sequences are called mutations – or markers. They are what make us individual, and they are also the means by which individual ancestry can be determined. Since these changes occur at a known constant rate over time – roughly once every thousand nucleotides – the greater number of differences between two people’s mutations, the further they are away from sharing a common ancestor.
Mitochondrial DNA (mtDNA) is inherited only through the maternal line in humans. This knowledge enabled geneticists to demonstrate that all humanity descended from a single female ancestor, now known as “Mitochondrial Eve,” and established that her birthplace, and that of all humanity, was Africa.
All male mammals have one Y chromosome that contains a gene called SRY, which triggers the development of a male. The Y chromosome is passed down essentially unchanged from one generation to the next; in other words, the Y chromosome DNA of all living men is related through a single male ancestor who lived 60,000–100,000 years ago. This discovery enabled population geneticists to trace human ancestries through the pattern of mutations or markers carried on the male Y chromosome.
Follow “The Journey of Man – a Genetic Odyssey” a 13-part PBS film of the amazing human journey out of Africa and across the world with population geneticist Spencer Wells.
We Were Not Alone Until Recently
Dr. Luca Cavalli-Sforza was a pioneer in using genetic information to trace human evolution, history and patterns of migration. He recognized the importance of synthesizing information from diverse disciplines – genetics, archaeology, linguistics, anthropology and statistics – to explain how human populations fanned out over the earth from their original home in Africa. David Reich, a professor of genetics at Harvard Medical School, and author of Who We Are and How We Got Here describes Dr. Cavalli-Sforza as the first scientist to predict that there would be “enough information in genes to determine where people came from in the world and who they’re most closely related to.”
The mapping of the complete genetic make-up (genome) of a human was first completed in 2003 and transformed the ability to retrieve DNA from archaeological material. Rapid advances in genetic technologies since then have enhanced our ability to identify remains in time and address many mysteries of the past, like the migration of people and the evolution and spread of culture through periods of time from hundreds to tens of thousands of years.
Today scientists can read billions of letters from the genomes of ancient humans and other organisms, changing our view of history and evolution. According to Johannes Krause, director of archaeogenetics at the Max Planck Institute for the Science of Human History in Jena, Germany, this genetic record is “like a lost library … and we’re just starting to learn the language of all those books that we have uncovered.”
A Record of our Hominid Ancestors
For anthropologists, too, the advantage of human aDNA is that it provides information that could not be gleaned from fossils or artifacts. Genetic evidence demonstrates that an archaic species branched from Homo erectus some 1.4 to 0.9 million years ago and then branched again between 770,000 and 550,000 years ago; one branch was anatomically modern humans and the other an archaic lineage that underwent a further division into Neanderthals and Denisovans between 470,000 and 380,000 years ago.
In his book Before the Dawn, Nicholas Wade details findings from aDNA through which we can track our evolution in Africa from ape to Homo sapiens, and map biological, physiological and behavioral changes more accurately than ever dreamt of prior to this recent ability.
The Benefits of Interbreeding
David Reich, Nicholas Wade, and Thomas Suddendorf all describe in their books how ancient genomes show definitively that our ancestors not only met but mated with Denisovans and Neanderthals – multiple times – between 100,000 and 40,000 years ago. This interbreeding conveyed genetic adaptations that improved our chances of survival.
We see evidence of this interbreeding in modern humans today, for example in an adaptation to high altitude inherited by ancestors of Tibetans who bred with one line of Denisovans; and in the ability of present-day Europeans and East Asians to withstand colder temperatures than do people from elsewhere, thanks to Neanderthal genes. By interbreeding with them, our ancestors inherited thousands of years of adaptation including immunities to local bacteria, viruses, and parasites. Scientists suggest that how some European people respond to the flu or to other viral infections today may well be influenced at least in part by their Neanderthal inheritance.
Tracking Human Movement Across the Globe
In the 1970’s Cavalli-Sforza used data from blood samples taken from people around the world with the idea that this would potentially establish a “family tree.”
In 2010 it became practical to sequence whole genomes from DNA extracted from ancient human bones, and to analyze the data to understand changes in biology over time. From this we learned that it is not possible to accurately reconstruct human history based on patterns of variation in people today because, as David Reich says, “people have moved and removed and mixed and migrated again and again and again.”
In his book Who we are and How we Got Here, David Reich describes how aDNA has given us an accurate record and timeline of population movements in Western Eurasia.
A major conclusion of this work is that all human populations today are mixtures resulting from multiple population migrations and gene flow. It also confirms, through the analysis of hundreds of West Eurasian ancient genomes, that the “Caucasian” (or white) racial grouping simply didn’t exist 10,000 years ago.
Prior to our ability to analyze aDNA, we could only interpret ancient history from art and artefacts. This led anthropologists and historians to assume that the spread of ideas was the primary cause of change. Now we know that the primary cause was often due not to influence but to interbreeding.
Two Large-Scale Migrations into Europe
Europe was massively transformed by two migrations, one after about 9,000 years ago which eventually brought near-eastern (Anatolian, Syrian, Iraqi) farmers to Europe about 6,500 years ago, where they interbred with local hunter-gatherers until about 4500–3000 BCE. The evidence shows that populations did not disappear, but mixed with each other, reducing the differentiation through expansion and intermingling.
5,000–4,500 years ago a new population arrived in Europe. These were Yamnaya steppe pastoralists who, herding sheep and cattle, spread across the steppes north of the Caspian Sea 5,300 years ago, travelling with newly-domesticated horses and carts on newly-invented wheels. They arrived in Europe with the Corded Ware Complex culture pottery and replaced other cultures and settlements, in some places as much as seventy percent. In Iberia the Yamnaya male population entirely displaced the local males, giving them preferential access to females for generations.
Burial sites found west of the Elbe from the Bell Beaker culture, contemporary to the Yamnaya, tell a different story. These sites first appear in Iberia about 4,700 years ago and another cluster is shown in Hungary at about the same time. However aDNA shows that there is almost no shared ancestry between these two populations, which means, unlike the Yamnaya, the spread had to be through the communication of ideas – most likely religious – rather than the movement of people. Once this culture reaches Britain and places further west, things change; and from about 4,400 years ago, the Bell Beaker people replaced the farmers almost entirely. Genetic evidence of the farmers, who were the last people to build Stonehenge, is no longer represented in the people of Britain today.
Indo-Aryan Migration Theory Upheld by aDNA Studies
More recent DNA evidence has shed new light on the origins of the Indian people, the Hindu religion and the Sanskrit language. DNA samples prove that there was a migration in the middle to late Bronze Age of the Eastern European Andronovo steppe peoples from Central Asia into India. They carried both Yamnaya DNA and Neolithic European DNA and likely spoke Indo-Aryan languages. Unlike the Yamnaya, the Andronovo peoples were very often blond and blue eyed. Their DNA shows higher rates of what we think of as European phenotypes such as tallness, fair-skin and blue or green eyes. These traits are found in people from North India, Pakistan and Afghanistan – such as the Nuristani and Kalasha people – but not in people from South India. The Andronovo were the first to ride horse-drawn chariots, which they did by about 2000 BCE. Sometime after this they invaded India from the North West.
Although no ancient DNA samples have been found within the Indus-Sarasvati Civilization sites, outlier samples from skeletons close by show a relationship to almost all Indians today but with no trace of Yamnaya DNA. This leads scholars to conclude that the Indus-Sarasvati civilization is the likely source of the many Dravidian languages spoken in the south of the Indian subcontinent, and it is certainly evidence of a later Yamnaya link to India from the Andronovo people.
How Biological Traits Evolve
aDNA has enabled scientists to understand how complex biological traits evolve over time. For example, by mapping changes in diet, scientists can trace the evolution of the ability to absorb vitamin D, or lactose tolerance, which enables us to drink milk into adulthood. They can follow the complex evolution of skin pigmentation, and traits such as blue eyes and height.
200,000 years ago modern humans were not behaviorally more sophisticated than Neanderthals. It was not until the Upper Paleolithic/Late Stone Age revolution about 50,000 years ago that a wealth of evidence of modern humans appeared, including complex tools – not just stone but bone tools – plus sewing, cave art and figurines. However, there is no evidence that changes in neurological genes accounted for this upsurge of complex behavior. It appears now, and aDNA evidence from our ancient ancestors shows, that human populations 50,000 years ago already had within them the genetic mutations for this complex behavior. This tells us that climate and the environment – particularly in Europe at that time – were the likely drivers.
Quick lessons to help you learn more about genomics, so you can explore your own data in a more meaningful way.
Researchers find that the appearance in Europe of pottery imprinted with cord-like designs may have been the result of intermarriage between Neolithic farm women of Europe and incoming warriors from the Pontic and Caspian steppes near the Black and Caspian seas.
Ann Gibbons, Science
A recent study shows that pale skin, as well as other traits such as tallness and the ability to digest milk as adults, arrived in most of Europe relatively recently.
Ian Sample, The Guardian
Traces of unknown ancestor emerged when researchers analyzed genomes from west African populations.
PBS NOVA ScienceNOW
Watch the amazing story of how the genetic history of lice gives us clues to mysteries of our own evolution.
The Powers, Perversions and Potential of Heredity
Our understanding of heredity has come a long way and holds much promise, but we’ll need wise judgement to manage the emerging science of genetic engineering.