Tracking down the human journey
“A journey of a thousand miles must begin with a single step.”Lao Tzu
It is no wonder that humans find humans a fascinating subject. Curiosity about one’s self origin arises not only due to narcissist impulses but also because we know that inquiring about our own heritage can help us to understand better who we are. In the collective side, there are many open questions about the first days of our species; one of them being, how we populated the Earth?
The main problem to trace human migration is, of course, the lack of information. The records provided by archaeology, might be a bit tricky, because once you realize that the funny rock is actually a fossilized femur, you have to date it and, even the finest dating techniques have some trouble going beyond 50,000 years. Yet, there are some events where most of the scientific community agrees that we, the Homo Sapiens Sapiens, appeared as such, around 200,000 years ago in East Africa.
DNA is perhaps the most powerful piece of information we have to unveil the human family tree. By comparing our mitochondrial DNA (mtDNA) we can know whether we come from the same female ancestor; this is because, in the absence of mutations, mtDNA is passed unchanged from mother to children, generation after generation. The same is true for the Y-chromosome, which is inherited from the father’s side. The rate at which the DNA accumulates mutations can be estimated; then, comparing DNA patterns between different people, it is possible to recreate the human lineages over time.
The latest (sort of) consensus estimates the original human population was around 2,000 people. One of the females in the group, now defined as Mitochondrial Eve, succeeded to procreate through her female-lineages, such that every living human nowadays descends from her. The male analogue is called the Y-chromosomal Adam, which is the male ancestor we all share. Despite the biblical names chosen by the scientist, neither the Mitochondrial Eve nor the Y-chromosomal Adam, were the only male or females alive. Actually it is likely that they lived in different times; what happened is that at some point their descendant met, mated and conquered the world. Perhaps the easiest way to understand it is looking at this random drift tree.
This tree represents five generations of woman. The top row is the oldest; there we can see that even when the Mitochondrial Eve (or the Y-chromosomal Adam) had contemporary females that also reproduce, after a few generations only her descendants survived.
In 2005, National Geographic launched the Genographic Project, which has collected DNA from more than half a million people across 140 countries. The aim of this project is to understand how we originated and how we populated Earth — a task that Prof. D. C. Wallace, from Stanford University and Prof. P. A. Underhill and colleagues from the Catholic University of Rome, Stanford, Oxford, Barcelona, Pavia and Atlanta University, started a few years ago.
Identifying different groups of mutations (from death and alive people), the geneticist can track different lineages over time. This is possible because (for instance) mtDNA accumulates mutations about every 3,500 years. The result of many scientists’ hard work, can be summarized in this map, published in the New York Times.
As can be appreciated in the map, the results support the theory that we all come from Africa and came out of it in different migration waves. Some lineages went to particular continents. Also, the results suggest that America was populated in a few different migration waves.
It is important to point out that these findings are “work in progress” and, as scientists collect more samples of DNA, this information might change. Even so, what we know so far is quite striking: everything points out that we all share a female and male ancestor. In the words of Prof. Wallace: ‘’What I have found astounding is that it clearly shows we are all one human family”- one very problematic family I must add, but one family after all.
It’s too hot, let’s go somewhere milder!
On question that genetics can’t answer is why the first humans decided to move? Intuitively we can guess the answer: as migrants nowadays, they probably were looking for a better quality of life, which could be translated in more sources of food and better weather. Most likely, it was a natural phenomenon that encouraged humans to explore further lands.
With the purpose of having the full picture of human migrations, geophysicists and physicists have tried to create a model that couples the “human diffusion” to climate variations.
Diffusion is ‘the spreading out’ of particles when they move from a high concentration area to a lower concentration one, or in other words, it is what happens to your choco when you pour it on milk. The simplest diffusive phenomena can be described with the continuity equation: it states that “a change in density in any part of the system is due to inflow and outflow of material into and out of that part of the system”. In a very abstract way, you can think of humans populating the world as particles of chocolate, with the important difference that we can reproduce and die — an effect that can be described using reaction-diffusion equations.
Prof. A. Timmermann and Dr. T. Friedrich from the University of Hawaii at Manoa, presented a model that describes the dispersion of humans “forced by spatiotemporal estimates of climate and sea level changes over the past 125 thousand years”. Their model used modified reaction-diffusion equations where “the growth and mortality of humans are dependent on three key vegetation and climate parameters: net primary productivity, which controls the availability of carbon-based food sources, desert fraction and temperature”. The challenge was to couple this model to orbital-scale climate simulations with enough detail to predict locally the changes in the geographic landscape, which were consistent with the archaeological records.
The result of their calculation is summarized in this video, it shows how climate variations created temporal (~12,000 years) “green corridors” across what is now the Sahara and Arabian desert. These paths were crossed by two migration waves around 106,000 and 73,000 years ago. After, we spread to Europe, Asia, and the rest of the world. This model could help to explain how and why genetic lineages diverged a long time, although in some events there is still discrepancy between both approaches. Archeogenetics and diffusion models could complement each other to give us the full picture.
This human diffusive model, resembles an idea that Isaac Asimov first presented in his series of books ‘The Foundation’. There complex events in society could be predicted using thermodynamic laws. Of course modern events like migration are far more complex phenomena; they have blurred boundary conditions and shorter time scale, while in the past the migration happen in periods spanning thousands of years (which made the system almost steady). Nowadays the migration waves are triggered by socioeconomic phenomena that can change drastically within just a couple of years. This makes the modern scenario quite turbulent. At least, now we have plenty of information about the people flux around the world, you can see it for yourself in a very cool interactive map created by the international organization for migration.
As our history has shown us, we are a family of travelers, before we were chasing the mammoths — now we chase jobs, safety, etc; after all, we are not so different from the first humans. Despite the natural and artificial borders, we students/migrants/expats/refugees/retirees, will keep moving to places where life seems better, diffusion can’t be stopped.
This article has been translated to Spanish.