Where Have We Come From?

Where have we come from?

This query has occupied human thought for many millennia; perhaps from the time human mind developed the capacity for analytical abstract thought. All ancient civilisations, and latter organised religions, had a unique myth of the origin of their race. For a very long-time people were unaware of the vastness of earth. Their immediate environment was their conceivable world. They could not imagine that they had come to their corner of earth from distant lands. 

The heroes and the villains in these myths were different, but all professed that their people were created by a divine being: Inti, the Sun god of Incas, Gaea and Uranus of Greeks, Marduk of Babylonians, Adam and Eve created by God of Christianity, Brahma, the creator, of Hindus.

Civilisations were born across the ancient world after the invention of Agriculture. People discovered land and sea routes to the distant corners of the earth and their inhabitants. But they remained smugly content in their provincial belief that the supreme God of the sky had ordained them and their ancestors to inhabit such a patch of land as they had occupied since the beginning of time. 

Charles Darwin upset this apple cart. In his earth-shaking magnum opus, The Origin of Species, he suggested that numerous species of living organisms were not created separately, but represented the ever proliferating and evolving branches of the tree of life. Thus, if traced back, a group of species will be seen to have originated from a bigger branch in the past. In an older past, groups of these bigger branches would converge on a still thicker branch. He postulated that if one went back sufficiently in the past, a single origin of all the millions of species on Earth, not unlike the trunk of a tree that is the progenitor of all its branches and twigs, will be found. 

For the religious beliefs of the past two millennia, The Origin of Species was a ground-shattering piece of work. Darwin was aware that it would quite simply pull the rug beneath the feet of the religion. Bereft of its singularly lofty purpose, i.e., creation of life, God would be left high and dry. Darwin didn’t want his theory to be drowned in the resulting brouhaha. In the book he spoke of all living beings but Humans. There is only one sentence about human origins in the book, perhaps the most understated truth in the history of the written word; ‘Light will be thrown on the origin of man and his history.’ He took this up in his latter book, The Descent of Man.

The light Darwin threw – and how! – on the origin of species, including Homo Sapiens, changed the course of human thought. People world over were forced to entertain a doubt about their creation in divine hands. One might still have chosen not to agree with Darwin - as many, perhaps a majority do even today – but none could disregard his theory.

In Darwin’s time no human fossil had ever been discovered, though Darwin speculated on their existence. The century that followed Darwin, witnessed a scramble for human fossils. Naturalists discovered them in every part of the world. Every discovery pushed our origin back in time, not by millennia but by hundreds of thousands of years. (At one time, a major crisis erupted for the Theory of Evolution. Evolution through the process of Natural Selection, is a very slow process, as was known to Darwin and the leading proponents of the theory. But the leading English physicist lord Kelvin, calculated the age of earth to be 20-200 million years – It is today known to be 3.8 billion years. This was grossly inadequate for evolution. Soon the fallacy in Kelvin's method was realized and Darwin’s correct explanation triumphed). The oldest and the maximum of human fossils were found in Africa. It became clear that modern humans originated in Africa and then dispersed over the rest of the world.

Palaeoanthropology, the study of human evolution and Archaeology, the study of human activity through analysis of material culture, gave large evidence of the dispersal of human ancestors out of Africa. But the time scale of these movements was riddled with inaccuracies. In recent decades, science of molecular biology, Genetics specifically, has vastly improved understanding of our origins.

DNA is the book on which is written the story of life, in a simple language comprising four letters and sixty-four words. Information on DNA is digital and hence lends itself to an incredible fidelity during copying, as individuals produce copies of their DNA to pass on to their progenies, stretching over thousands of generations. A copying error – known as mutation – occasionally slips in during replication of DNA. Further progenies carry this error, the mutation, faithfully. Mutations accumulate at a constant rate, which is specific to a gene. Thus, higher the number of differences in a particular gene in two species, deeper in the past they shared a common ancestor. Through this marvellous genetic clock, the branching family tree of living organisms comprising all the living and extinct species has been charted. And it tells that every one of the living species – Humans, Whales, Tigers, Snakes, Fishes, Birds, Yeasts, giant Redwood trees  – originated from a single celled bacterium about 3.5 billion years ago. This is the dizzyingly unifying truth of life. Millions of amazingly endowed living species and many millions of equally gifted extinct ones owe their origin to the spark which transformed an inert collection of few chemicals into a reproducing blob of organic matter.

Molecular clocks are highly suitable to chart the family tree of species as individuals of different species do not interbreed. Differences in a gene between two species are only due to accumulated mutations, since the time two species separated from a common ancestor. Sex plays havoc with the information in the archives of DNA. Every sex cell is a jumbled-up bag of information containing bits of genetic sequences from both the parents. Each individual carries randomly chosen genes from their numerous ancestors. It is exceedingly difficult to decipher the dispersal and racial origins of the modern human populations - all belonging to one species, Homo Sapiens, with freely interbreeding individuals -  through genomic differences.

Mitochondrial DNA, mtDNA, comes to our rescue. Mitochondria are the minute powerhouses of cells, where energy required for cellular function is produced. They have a unique property; they carry their own DNA. This DNA doesn’t mix with the nuclear DNA of the cell or with other mitochondrial DNA during cell division. We inherit mtDNA only from our mother, who inherited her mtDNA from her mother. Thus, inheritance of mtDNA travels only in the maternal line of an individual. Similarly, the Y-chromosome, which is inherited solely from one’s father, can be used to trace paternal ancestors of an individual. It should be remembered that mtDNA and Y-chromosome, tell the history of only these two strands of DNA. Humans inherit DNA from many ancestors. And a specific lineage can be built, at least theoretically, for every gene. 

Analysis of mtDNA of diverse people from around the globe in the year 1987, was used to construct the family tree of maternal relationships of modern human populations. It was found that the deepest branch of this tree – the branch that left the main trunk earliest, at about 1,60,000 years ago – is found today in the sub-Saharan people of Africa, suggesting that the ancestors of all modern people lived in Africa. This grand ancestress of today’s humanity was sentimentally named the African Eve or the Mitochondrial Eve. One must beware of a misunderstanding. It is not that the African Eve was the only woman in her time and we all are her descendants. There were thousands of others and descendants of many are among us today. But female descendants of these women ultimately petered out, while their male descendants have continued to successfully pass on their genes. Mitochondrial Eve is our most recent common ancestor in the purely mtDNA line.

Modern humans, carrying the mtDNA of the African Eve, migrated out of Africa around 50,000-70,000 years ago. They spread over Asia, Oceania, and Europe between 40,000-50,000 years ago. Migration of people across the globe continued for a very long time after this. Modern people inhabiting a country today are the descendants of these ancestors and have mixed inheritance of many races.

In the past molecular scientists had recourse only to mtDNA and Y-chromosome, to trace the ancestry of modern populations. These are inadequate to trace our history in the last 10-20,000 years.

In 2001, human genome was sequenced for the first time. In the next decade the cost of reading DNA sequence in a genome came down enormously. This gave a huge fillip to the studies of human past. Each individual carries genes inherited from multiple ancestors from many races in the past. Whole genome sequencing, in contrast to sole mtDNA or Y-chromosome analysis, informs us about the contribution of these ancestral lineages in modern people. Around this time the process of extraction of DNA from fossil bones also improved vastly. Ancient DNA studies have now revealed startling details about population movements in the deep past. 

‘We have found the secret of life,’ Francis Crick exclaimed on 28 February 1953, as he walked into the Eagle pub in Cambridge, UK, with his collaborator in this quest, James Watson, as quoted by the latter. What they discovered is not only the secret of life, but also a book of the origin of living beings – their history from the time life began about 3.5 billion years ago. Watson and Crick deciphered the language of this genetic book in 1953. Science has now discovered newer meanings hidden in its passages. They reveal a mesmerising story. 

Origin of people in a particular country is a subject beloved of politicians. It incites a frenzied patriotism among citizens. Demagogues cleverly manipulate public opinion in the aftermath of reasonless passion it generates. Truth is mute. It is subtle. It is hidden, quite literally, under the misleading detritus of time, time measured in millennia, in thousands of millennia. But truth comprises facts, far staggering than human imagination. History of our ancestors, engraved on our DNA, tells a story of stark simplicity and captivating beauty. One must not allow ideological bigotry to deprive oneself of the joys of this discovery.

PS: In a later post, I will attempt the story of our ancestors as written on the genes of us Indians. I had started the piece with this intention. Then the introduction grew longer and longer - though I had dealt with most points with extreme brevity. And I couldn't see a way out without surrendering clarity.