Storytelling Science

What do you know about your 10,000th grandmother?

Amitabha Mukerjee


Picture yourself as a child, holding your mother's hand - you may even have a photograph of this. Now picture your grandmother, holding your mother's hand. And with a leap of the imagination, you can see your great-grandmother holding your grandmother's other hand, and so on ... all your grandmothers in a line, stretching into eternity. Your great-grandmothers lived at different times, but they each held their daughter's hand, and, in turn, were held by their mother, so it is a plausible image!

Now, count your grandmothers. Starting from your mother as one, your grandmother is two, and your great grandmother is three, and so on. How much older than you is your mother? Thirty years is a good average for today, but as we go back, the average age difference may have been less. By the time of your tenth grandmother, you are probably two hundred and fifty years back, and the Battle of Plassey where the British consolidated their foothold in India, is just about to be fought.

Going back further yet, your hundredth grandmother lived more than 2000 years back, in the time of the Mauryas. What kind of clothing did she wear as she held the hand of her daughter, your 99th grandmother?

Your Ten-Thousandth Grandmother

If we were to go further back - all the way to your ten-thousandth grandmother... It turns out she is not even a "modern" human being. Living more than 200,000 years ago, she would have looked quite different: her face was shorter, with prominent ridges at the eyebrow. Also, her pelvis was smaller so that her baby's head at birth must have been smaller (that is your 9999th grandmother's head!)

The species of modern human beings, Homo sapiens sapiens, emerged about 130,000 years ago in Africa. However, your 10,000th great grandmother (let's call her Prachinnaa), was a Homo Erectus, a species that emerged about 1.8 million years ago and became extinct some hundred thousand years ago.

But did Prachinaa live in Africa, or in India?

This has been the source of considerable debate. On one side is the "Out-of-Africa" theory, which says that the Homo sapiens migrated and replaced existing Homo Erectus populations outside Africa. On the other side was the "Multi-Regional" theory, which said that groups in different regions evolved separately, and then intermixed, so that our present genetic pool is largely uniform. By the first, Prachinaa lived in Africa, whereas according to the second, she may have lived in India.

We now turn to my friend Vinayak Eswaran at IIT Kanpur, who had something important to say on this matter recently.

Ink diffusion in water

Take a glass of water and a sketch pen. For a second, dip your pen into the water. Immediately, there is a small intense point of colour, but then it spreads or "diffuses" out across the water: this process is controlled by an equation called the "diffusion equation".

All his life, Eswaran, as a Mechanical Engineer, has been studying the mathematics of diffusion, which is useful in engineering because it determines the dispersal of heat, as well as fluid intermixing.

Time and again, human creativity is shown to be nothing but the application of one set of ideas, from one domain, into another completely different area. I mean, how different could Mechanical Engineering be from questions about Prachinaa? Yet, his Engineering perspective allowed Eswaran to look at this problem in a dramatically different way.

Modern human genes came out of Africa, and through interbreeding, diffused into pre-existing Homo Erectus populations in the rest of the world.

What Eswaran did was to take the standard assumptions of the anthropologists, and put numbers to the theory using a population dispersion analysis. The scenarios that match the historical evidence best are most likely to have actually happened. In this way, he was able to show that an anatomically modern human, with a small evolutionary advantage - for example, a larger birth canal resulting in fewer deaths of newborns or mothers, might gradually replace archaic humans. This gene would propagate by interbreeding with native populations, gradually replacing all signs of the older gene with the newer, biologically advantageous gene:

"The simulations show the wave front to be barely 800 km in width, and the region within which clear signs of hybridization would have appeared could have been as narrow as 300 km. Hybrids ahead of this narrow region were close to archaic, while those behind were essentially modern."
Thus Eswaran's solution proposes to combine both theories. According to him, Prachinaa was not quite a modern human, and she may have lived either in India, or somewhere not too far away. However, at some point, over a period of two to three thousand years, her descendants changed into the genetic pattern of Homo sapiens, which is what you are today.

Eswaran's wave diffusion model was published in 2002, but immediately diffused rapidly through anthropological literature. Recently, it has been used to explain the evolution of lice across the continents which appeared to have diffused along with the modern humans). Arguably, Eswaran is now better known as an anthropologist!

Stand in front of the mirror, and look at your large head. Think of the large brain inside it. And then thank Prachinaa and all your other grandmothers who endured through the millennia, so that you could be alive today.