Out of Eden: The Peopling of the World (51 page)

I have stuck my neck out to place modern humans in Malaysia by this date on the basis of the Kota Tampan site where tools were found under a thick layer of volcanic ash from Toba. The key tools were indisputably artefacts, and the ash did come straight from the sky 74,000 years ago. But in spite of majority view that the Kota Tampan tools were the handiwork of modern humans, they could still theoretically have been made by other humans, since no bones have been found on-site which would confirm the identity of their makers. The only modern human remains of that antiquity found in the region are the now re-dated Liujiang skull and partial skeleton from southern China. The dating of the earliest Flores (Eastern Indonesia) occupation by modern humans remains to be published.

I have several corroborating reasons for relying on such a shaky lithic connection. First of all, the logic of the low-water colonization of Australia 65,000 years ago fits; and second, increasing numbers of genetic dates outside Africa easily reach back to this time. The next available low-water slot for the colonization of Australia would have been around 50,000 years ago, but that does not fit the other evidence so well.

The trouble is that it is difficult enough persuading a majority of Australian archaeologists that Australia was colonized by 60,000 years ago, let alone 65,000 years. The problem with suggesting any dates for modern humans outside Africa much beyond 45,000 years ago is again the radiocarbon ceiling. If one were to rely solely on radiocarbon dating, the whole human world would seem to have
started just over 40,000 years ago. Only a few Australian dates of greater antiquity (e.g. in the region of 60,000 years) have been obtained by other methods, and they have so many problems it is easy to rubbish them. We could take the view that the colonization date for Australia just happens to coincide with the radiocarbon ceiling, but that is likely to be wrong. The safest thing, of course, is to say the date of first colonization is unknown. In this speculative synthesis, however, I would rather go with an earlier date which fits Kota Tampan and low sea levels than one which is more precise and recent, but eventually turns out to be wrong.

Toba is also regarded by some as having caused worldwide population extinctions as a result of the ‘nuclear winter’ that followed. I have taken this into account in my reconstruction. India bore the brunt of the massive ash fall, and may have suffered mass extinction, since the Toba plume spread north-west across the Indian Ocean from Sumatra. This may explain why most Indian M sub-groups are not shared elsewhere in Asia and the dates of the M clan’s re-expansion are paradoxically younger in India than elsewhere in Asia and Australasia.

What of the future?

In my view it is not quite as rosy as some would hope. We are like all other species in that our evolution and survival are both subject to the controlling and moulding influence of the changing environment, whether the changes result from the ice-age cycles or from our own prodigal exploitation of the planet’s resources. One of the lessons of the past is presented by the longest timescale, that of recurring glacial maxima. Eventually, it is certain, there will be another freeze-up, at which point our species will be stressed and may well hit another bottleneck. It is hard to overstate the bearing that ice-age cycles have had on our fate on the planet. Taking the long view, the effects of global warming could be little more than a blip on the way to the next glacial maximum.

However, in the short term there are more telling pressures. A key consideration throughout our story has been genetic diversity; it is, after all, the slowly evolving diversification of mitochondrial DNA and Y-chromosome lines that has been helping us to trace our past. What has not been emphasized so far is the fact that, while we are still recovering from the last ice-age cycle and previous bottlenecks, our
total
genetic diversity remains relatively low. Our lack of diversity as a species lays us open to new pandemics of infection in crowded, interacting communities.

Diversity, diversity, diversity

Here’s an anecdote. When I was working in Hong Kong, one of my senior lecturers was a charming and sincere Taiwanese paediatric cardiologist. She asked me once what the difference was between killing a tiger for its body parts and slaughtering cattle. This was not an insensitive question. She did not personally take any tiger-bone medicine and felt sorry for tigers and their loss, because they were such beautiful animals. In short, she disagreed with the trade; but she was not convinced by the Western-imposed philosophical argument that the act of killing was immoral for one large animal and not for another. Nor could she see from a logical, detached point of view why it is necessarily more important to preserve a large, and to us beautiful, mammalian species than, say, a rodent.

I scratched my head for an appropriate philosophical answer to the question as posed, an answer which would avoid the issue of whether we had the right to take animal life at all, and would also avoid invoking the aesthetic appeal of big cats. Eventually, I think, I persuaded her that the objective difference between killing members of the two large species, one wild and one domestic, from the perspective of conservation was in preserving biological diversity or variety.

Domestic cattle, as a whole, have very little of their ancestral (aurochs) genetic diversity left, but they have millions of copies of
that remaining diversity. Domesticating a species always reduces its overall diversity, though it may introduce variation in specific qualities such as size. By contrast, each of the few thousand remaining wild tigers still holds a significant proportion of the original diversity of their various races, so any one tiger is a unique genetic treasure house for the species and therefore much more valuable than one cow. So, apart from the fact that tigers are at greater immediate risk of extinction than cows, killing one tiger also affects a much greater proportion of species diversity than killing one cow.

Another big cat, the cheetah, is at even more at risk of extinction than the tiger. This is not just because of low numbers, but because there is virtually no diversity left in wild cheetahs – they are nearly all related to possibly a single pair that survived the last ice age. Surprisingly, non-Africans are closer to cheetahs and cattle than they are to tigers in respect of their genetic diversity, since they can all trace their lines back to a few mothers and fathers who left Africa only 85,000 years ago.

So what is so important about diversity, apart from the aesthetic aspect of variety? The answer is survival. Random diversity is nature’s and evolution’s fuel depot. Without randomly generated genetic diversity, species lack the flexibility to survive and adapt to the various stresses imposed upon them. Random diversity takes many generations to develop from a single breeding pair, so species which have gone through a tight genetic bottleneck have a lot of catching up to do.

Killer disease

It may come as a surprise to hear that we are still under constant evolutionary stress. The best and most important example of an ever-present and ever-changing evolutionary stress is infectious disease. Bacteria and viruses evolve much faster than we do. To combat new varieties of disease which bugs evolve to ‘invade’ us with, we each have built-in diversity in our immune system to enable us to
identify new varieties of bugs and set up a specific defence. The capacity of the body to recognize and combat a variety of different infectious diseases is genetically determined. The diversity of immune response held in each of us has limits, however, and depends partly on the particular bugs that our own community has met in the past.

Most of such genetic variation in resistance to disease operates through the adaptive immune system. Some populations appear to have a sounder immune response to certain diseases which may have afflicted their ancestors in the past. I came across an example when I was working in Hong Kong, where ethnic Chinese children almost never fall sick with meningococcal disease (meningitis and/or septicaemia). They usually develop detectable specific immunity to meningococci in the blood but, unlike Europeans and other non-Chinese groups, they completely avoid the disease and also do not act as carriers for the bug. In contrast, the commonest organism to find in Hong Kong Chinese with meningitis is the tuberculosis bacillus, which is extremely rare as a cause of meningitis in other developed populations. This implies that there are differences in the quality of aquired immunity to specific diseases between different modern populations.

Against other infections such as malaria, our innate defences are not solely immunological. Certain genetic disorders common in the tropics, as a result of evolutionary selection, directly impair the successful multiplication of the malarial parasite. This mechanism of genetic protection against disease in the case of malaria, one of the greatest killers, lies in inherited disorders of red blood cells, where the parasite seeks to make its home. These genetic disorders, a large proportion of which go under the general name the ‘thalassaemias’, are common in regions that suffer or have suffered malaria in the recent past. The name derives from Greek ‘θαλασσα’ for the sea, since some Mediterranean islands such as Cyprus have high rates of
these diseases. These disorders however occur throughout tropical and sub-tropical regions.

As the host evolves more appropriate defences for the bugs, however, the bugs are themselves busy evolving to get round the new defences. The trouble is their evolution is faster than ours. The smartest are those that do not kill their host. Unfortunately not all bugs realize this. The evolutionary leapfrog race between infectious disease and animal hosts often takes a bad turn for the host when the bugs jump from one species to the next. Some of our most virulent viral and bacterial diseases, including bubonic plague, emerge from animals living in the wildernesses we invade. Another such hitchhiker is the human immunodeficiency virus (HIV), which is now overtaking tuberculosis and malaria as the modern captain of death.

There is a self-comforting myth that Aids is a one-off ‘bad luck’ plague, uniquely super-lethal because it attacks the immune system, and its like will not appear again. Not so: it is a warning of the complex opportunism of infectious disease, which we will meet more and more as we expand into the corners of our shrinking world. In any case, the two varieties of the virus, HIV1 and HIV2, may derive independently from two different African primate species. In turn, our increasing fondness for intercontinental travel and fraternization helps spread diseases which, in the past, might have remained localized. As I write, a new infectious disease going by the acronym SARS has arisen in South China and simply adds to the list of exotic and serious infections that may have arisen in animals; it has then subsequently spread internationally from human to human aided by the jet plane.

Genetic intervention

One answer to the increasing threat of pandemic disease may seem to be genetic intervention. Every few weeks, documentaries and newspaper stories tell us of advances in genetic prediction and intervention which are going to change our lives and those of future
generations. Genetic disease will be eradicated, we are told, and the more well-heeled in our societies will be able to specify designer babies or pick a clone off the shelf.

Prenatal diagnosis and genetic counselling services for lethal and serious genetically determined blood disorders have been around for some time and make an enormous difference to the lives of individuals. They have had a major influence in countries such as Cyprus, which suffers high rates of beta thalassaemia mutations as a result of previous malarial selection. Such ethically motivated interventions will continue to prevent individual misery.

However, what genetic intervention cannot do is increase our collective genetic diversity. Putting aside the ethical and technical problems, the fact is that genetic intervention can only reduce diversity. This applies equally to the abhorrent concept of culling the more subjective ‘undesirable’ genetic elements, as practiced by the Nazis on a variety of patients with mental and other disease as well as on Jews and gypsies, and to the
Brave New World
concept of designer humans. Even if a new breed of geneticists were able to design an especially ‘good model’ that found a large market among potential parents, the exercise would be self-defeating. A clone of such superhumans expanding in our midst would reduce our herd diversity, thus increasing our herd susceptibility to new infectious diseases.

Interestingly, the two characteristics of our species which interest us most, our brain size and our longevity, are potentially amenable to simple genetic interference. The former more than the latter. It seems likely that if scientists were allowed to, they could ‘make’ a human with an even larger cerebral cortex within decades, based on present knowledge. This could be either by manipulating single homeotic genes (genes that control the organization of the embryo and body organs) or more crudely by injecting the product of that gene at an appropriate time in embryonic development. Whether a
genetically engineered ‘big-brain’ would be wiser or more intelligent, I do not know. I hope not to survive long enough to see it.

As far as longevity for the rich is concerned, there may be some business opportunities . . . – but there are also warnings. Ira Gershwin had a nice, though politically incorrect, take on Methusaleh’s longevity in the lyrics of the song ‘It Ain’t Necessarily So’ (from George Gershwin’s Porgy and Bess): ‘But who calls dat livin’ / When no gal’ll give in / To no man what’s nine hundred years?’ And there is the risk of overcrowding. According to a Vietnamese origin story, humans originally gained immortality by burying their dead under the tree of life. One day the lizard, who was fed up with having his tail trodden on by the crowd, suggested burying the dead under the tree of death. Life became easier after that.