The Journey of Man: A Genetic Odyssey (17 page)

What can we say about the order in which the ingredients were added? Well, the first pattern is that all of the soups contain impala. This means that the most likely explanation is that the original soup also contained impala – much more likely than if all of the cooks had independently decided to add impala at some point in the past. Remember that this ingredient is extremely difficult to obtain in most parts of the world. The next obvious pattern is the one we noted before: some recipes contain salt while others contain pepper. By the same reasoning that placed impala first in the ingredient list, because it minimizes the number of independent, identical ingredient changes, salt or pepper define the next addition to the recipes. After that, we see that mustard unites two of the recipes, making it the next addition after black pepper. So, we have derived an order for the addition of ingredients by our ancestors: impala, followed by salt/pepper, followed by (on the pepper lineage) mustard. Order emerges out of chaos.

In the case of our soups, it may seem that we could have chosen to ignore the possibility that the same ingredient could have been added independently. Why couldn’t mustard also show up in the salt recipes, for instance? While this may be possible for common ingredients, we should actually imagine the soup components as rare, home-grown varieties produced in small batches, only available in tiny local shops. In this case, it is virtually impossible for a cook from Mexico and one from Namibia to use the same type of mustard – our sophisticated palates would be able to tell the difference. Adding the same ingredient independently is almost impossible.

This sort of puzzle-solving is exactly what we have done for the markers that define our genetic lineages. If M168 defines a marker common to all non-African populations, then in our genetic recipe it is the equivalent of impala – the marker that unites everyone outside of Africa. If the lineage then splits into salt and pepper, we can imagine M130 – our Australian marker – as genetic salt, while pepper is represented by another marker, known as M89. Because of the order in which the ingredients have been added, we can infer that M130 and M89 are approximately the same age. Since we know that we were in Australia between 50,000 and 60,000 years ago, and M130 has not been found in Africa, we can set this as the upper limit for the age of these markers – it is likely that they arose at or after this time. The
archaeology gives us an independent way of assessing how old they are. But what if we wanted to guess at the age using only genetic data? Could we do it?

The answer is yes, and this brings us to the other dating method – counterpart to the relative dating we used to assign the order of the ingredients. Like the isotopic dating methods discussed in
Chapter 4
– particularly the ones with the intimidating names – the possibility of error is high for absolute genetic dating methods, because there are quite a few assumptions involved in the way the dates are calculated. Nonetheless, they provide us with an assessment of the age of markers – and therefore people – that is independent of the archaeological record. To see how this works, we will use our soup recipes to try to figure out the absolute ages of the ingredients – in other words, the time in the past when the ingredients were first added to the recipe.

The first rule for absolute dating, as mentioned above, is that the ingredients are added at a regular rate. The second is that once an ingredient is added, it becomes a permanent part of the recipe – there is no way to remove it later if you don’t like it. From these two rules it’s easy to predict that, over time, the soup recipes should become more and more complicated. The longer they have been accumulating ingredients, the more culinary diversity we should see. And because the ingredients have been added by particular people living in the past, they are like a culinary signature of our ancestors. They mark not simply the ingredients, but the people who added them. So, by dating the ingredients, we actually date the cooks who passed on our recipes.

Let’s assume that the ingredients are added at a rate of one every ten generations. Most people are happy to cook the same soup their parents did, but some finicky person pops up every ten generations or so who has to change the recipe in a minor way in order to ‘improve’ it. We can use this to estimate how many generations ago our first impala soup was prepared. There are four additional ingredients in each of the soup recipes shown above, so we have been accumulating changes for around forty generations (4 × 10). If we assume that there are, on average, twenty-five years in each generation (the average age of parents when they have children), this gives us a time of 1,000 years that the recipes have been accumulating changes. Therefore, the person who started cooking soup with impala also lived about 1,000 years
ago. We can even, by looking at where the ingredient occurs, guess at where this person may have lived. If we assume that newly added ingredients are chosen locally, then where would that person most likely have been living in order to choose impala? Since impala are an African species, Africa is the most likely place.

So, by looking at the soups and making a few assumptions about the way they change, we’ve been able to do two things. We have derived an order for the addition of the ingredients, and we’ve been able to estimate the time and place when the ingredients were added. In other words, we have used a bit of tasting and mathematics to tell us the
who, where
and
when
of soup history. Rather amazing, really – to be able to say so much from a few tastes.

In the same way that soup tasting can give us a glimpse of the culinary past, so too can genetic ‘tasting’ – which we call sampling – tell us about the human past. By inferring relative and absolute dates, and asking where the most likely origin would have been, we can actually trace ancient genetic migrations around the world. The first stop is at the edge of an ebbing Mediterranean wave, just before the world dried out and trapped a few people in the Middle East, around 45,000 years ago.

As we have seen, the Middle East has always been an extension of north-eastern Africa, to both grazing animals and the humans that hunted them. This had been the case millions of years before, when
Homo erectus
moved into the Caucasus via the Levant soon after he appeared in Africa. Between the hominid homeland in the Rift Valley and the benign Mediterranean climate, however, lies the eastern edge of the Sahara Desert. This gives a clue about the time and the route that we can use to test our genetic estimates.

Major geographical features – seas, deserts and mountains – have always served as barriers to the dispersal of living organisms. The unique flora and fauna of Australasia, for instance, have been maintained by the presence of an unbroken water barrier between this continent and the rest of the world. Similarly, mountains can act as
barriers, incongruous pieces of arctic real estate that serve to deter movement. In a way, geographical barriers are like bollards – those raised reflective markers that serve to guide automobile traffic.

While seas and mountains are (at least on the time scale of human evolution) huge barriers to movement, deserts are much more fluid. As we have seen with the forests and savannahs of Africa, deserts are interchangeable with other ecosystems. If the rainfall drops below a certain level, desertification can happen nearly overnight. Similarly, increases in rainfall can reclaim fertile land from the sands just as suddenly. Because of this, deserts should actually be seen as ebbing and flowing ecosystems, extending their range when the climate is dry and losing it when moisture is more plentiful – like waves lapping at the edges of the other ecosystems. Paraphrasing that old saying about British weather, if you don’t like the desert, simply wait a few hundred years and it will change.

The largest desert in the world is found in Africa: the Sahara. It evokes images of rolling dunes, camels, oases, date palms and extreme heat – the name is almost synonymous with desert. It has served as an extraordinary barrier to human movement throughout recorded history, to such an extent that Africa is divided by geographers in two zones: Saharan and sub-Saharan. The Saharan region has historically been closer to the Mediterranean world, since human settlement was limited to a narrow strip along the coast. The sub-Saharan zone, well beyond the Pharaonic sixth cataract of the Nile, was a distant and mysterious place, isolated by a 2,000-km wide strip of sand and heat. Clearly a significant barrier.

The Sahara has not always been like this, however. During the early stages of modern human development, it was a relatively moist place, with a significant human presence. Middle Palaeolithic sites dating to 80–100,000 years ago have been found throughout, and it is only with the acceleration of the last ice age after 80,000 years ago that humans disappear from the Sahara. There appears to have been a short ‘spike’ of elevated temperatures (and thus increased rainfall) around 50,000 years ago, when the northern hemisphere warmed slightly for a few thousand years, but the general trend from 70,000 years ago is one of lower and lower temperatures. In the case of Africa, this meant drier conditions and an expanding Sahara. We know this because of an
increase in sand in the sediments from the Mediterranean during this time, as well as the disappearance of savannah species from the desert itself.

The first Upper Palaeolithic humans may have reached the Middle East during the relatively warm and moist conditions around 50,000 years ago, when the eastern Sahara was in retreat and a gateway opened along the Red Sea. Perhaps they migrated down the Nile to the Mediterranean, then spread eastward across the Sinai peninsula. Alternatively, early human populations may have moved across the strait of Bab al Mandab into southern Arabia, a short hop of 20 km or so. Once there, the relatively moist conditions along the coastal mountain range of western Arabia – which served to scoop moisture from the prevailing westerly winds coming off the Red Sea – may have created savannah-like hunting conditions for these Upper Palaeolithic people. Even today there is a narrow strip of steppe extending as far north as the city of Medina in Saudi Arabia, unique in the harsh environment that defines most of the Arabian peninsula. In the past, this tenuous steppe environment may have been joined with its ecological equivalent extending southward from the Gulf of Aqaba in Jordan, effectively opening a door to the interior of Eurasia.

William Calvin, a neurobiologist who has written extensively on climate and early human evolution, has compared the Sahara to a kind of hominid ‘pump’. During wetter periods, the Sahara would have sustained human populations, perhaps focused around oases or rivers, or limited to zones that received moisture from prevailing winds. As the conditions turned drier, the Sahara would have returned to uninhabitable desert, forcing human emigration. Calvin suggests that the climatological downturn after 50,000 years ago may have been the impetus for the migration of Upper Palaeolithic humans out of northern Africa and into the Middle East.

However the earliest Upper Palaeolithic moderns reached the Levant, it is clear that the deteriorating climate after 45,000 years ago effectively locked them into their new home. The Sahara would have been at its driest between 40,000 and 20,000 years ago, and it is likely that any previously inhabitable areas there would have been engulfed by desert during this time. Modern humans were trapped in a new continent.

The genetic pattern bears this out, and provides the next clue on our journey. M89, the marker that occurred immediately after M168 on our main line into Eurasia, has been dated using the absolute method detailed above to around 40,000 years ago. Due to possible errors in the assumptions that go into the calculation, particularly in determining the rate at which new mutations occur, this estimate actually encompasses a range between 30,000 and 50,000 years, and it is likely (given the climatic data) that it appeared at the earlier end of this range, perhaps 45,000–50,000 years ago. This is because it serves to unite populations living in north-eastern Africa – Ethiopia and Sudan in particular – with the populations of the Levant. The shutting of the Saharan gate after these M89-bearing populations were allowed through is suggested by the low frequency in north-eastern Africa of Eurasian markers that occurred
later
on the M89 lineage. If Africa and the Levant had been part of a continuous range occupied by humans throughout the Upper Palaeolithic, we would expect to see a relatively homogeneous distribution of markers throughout. In fact, it seems that the emigration of populations bearing M89, which we can call a Middle Eastern marker, signified the last substantial Upper Palaeolithic exchange between sub-Saharan Africa and Eurasia. The world had been divided into African and Eurasian, and it was to be tens of thousands of years until significant exchange was to take place again.