The Journey of Man: A Genetic Odyssey (11 page)

When I was a child, my friends and I used to play a silly quiz game with each other, where we would ask trick questions intended to show off our command of obscure facts. One of the favourites was to name the largest island on earth. The naïve answer was ‘Australia’, which would always elicit a groan of disapproval. This is because Australia, as the groaners knew, is part of the continent of Australasia – not simply a large island. Encompassing Australia, New Zealand, Tasmania, New Guinea and several of the easternmost Indonesian islands, Australasia is the ‘odd man out’ in the geographic mnemonic stakes. And what an odd continent it is.

Present-day Australia is the driest subcontinent on earth – more than 90 per cent of it receives less than 1,000 mm of rainfall per year. Partly as a response to the environmental challenge of living there, it is the most urbanized nation in the modern world, with 90 per cent of its population living in cities along the coast. It boasts the planet’s longest continuous coral reef, the awe-inspiring 2,000-kilometre Great Barrier Reef. Perhaps the most interesting thing about Australia, though, is its fauna. The animals in Australia are unlike those anywhere else on the planet, with the only similarities found to those of New Guinea – also part of Australasia. The reason for this uniqueness is the extreme isolation of the place. Anyone who has sat through the two-night flight from London to Sydney can attest to the difficulty involved in getting there. Through the vagaries of plate tectonics, Australia has been disconnected from the continents of Eurasia, the Americas and Africa for the past 100 million years or so – its most recent connection was to Antarctica! What this isolation has meant is that Australia has missed out on most of the main line of mammalian evolution, with its wealth of placental species. The lack of ‘normal’ mammals has allowed evolution to pursue a different path, resulting in oddities like the platypus and the kangaroo. It has also meant that, until quite recently, Australasia had no primates – no monkeys, no apes, not even a bushbaby. Humans are the only primate species on the continent.

The lack of evolutionary antecedents means that humans must have colonized Australia from somewhere else. But where did they come from? The journey clearly involved a significant sea voyage, even from its nearest continental neighbours. If we allow for fluctuations in sea level as a result of climatic fluctuation, the landmass of Sahul (which included New Guinea and Tasmania, in addition to Australia) that was created during the coldest part of the last ice age, approximately 20,000 years ago, would still have been approximately 100 km from the rest of south-east Asia. The answer to how and when Australia was colonized by humans is one of the key pieces in our effort to solve the puzzle of how modern humans settled the world. The details it reveals about human history – and the methods of analysis involved in piecing it together – will set the pattern for the rest of our journey.

Lake Mungo, in New South Wales, is about 1,000 km west of Sydney. From the nearest town with an airport, Mildura, it is a 120-km drive on a dirt track through the hot scrub desert that comprises much of inland Australia. Mungo is no longer a lake – the water dried up over
10,000 years ago, leaving behind fantastic sand and clay formations that are reminiscent of those at Mono Lake in northern California – but between 45,000 and 20,000 years ago it was part of a lush oasis known as the Willandra Lakes. The lakes were fed by the Willandra Creek, which joined the Murray River further south, and ultimately emptied into Encounter Bay near present-day Adelaide. From the animal remains found at the site it is clear that several large species of extinct marsupials lived around the lakes, including the buffalo-sized
Zygomaturus
and a 200-kg short-faced kangaroo,
Procoptodon.
All of the animals of the area were herbivores, and as such they would have been tempting prey to humans.

It was around the earlier end of this time range, according to recently obtained dates, that a man was buried there. Called Mungo 3 by his discoverer, Jim Bowler, the find was dated to around 30,000 years ago when it was discovered in 1974. More recent dating methods have pushed the age back to 45,000 years, and human artefacts from sedimentary layers below Mungo 3 hint at dates as ancient as 60,000 years before present. If confirmed, these dates will make Mungo the earliest site in the world outside Africa to be inhabited by anatomically modern humans.

The earliest human remains in Australia, like those elsewhere in the world, have been dated using isotopic decay methods. These methods measure the ratio of different isotopes of an atom present in the sample. It is possible to do this because almost all atoms come in more than one ‘flavour’, depending on how many subatomic building blocks (particles called neutrons) they have. Through the alchemy of particle physics, the ‘heavier’ atoms tend to shed some of their particles over time, in the process transforming them into the ‘lighter’ atoms. By knowing the rate at which this decay occurs, and measuring the ratio of the heavier to the lighter atom, it is possible to calculate how long the decay has been going on. Like the molecular clock discussed in
Chapter 2
, this atomic clock provides critical time estimates for the study of ancient human remains.

The most widely applied form of isotopic dating is the so-called radiocarbon method, which measures the ratio of Carbon-14 (C-14) to Carbon-12 (C-12) in the sample. C-14, through a complex interaction with the atmosphere, breaks down to Nitrogen-14 (N-14). The
rate of breakdown depends on the so-called half-life of C-14, which is the amount of time required for one-half of the C-14 in a sample to decay – around 5,700 years. Since carbon is used to build organic molecules, like those found in plant and animal tissues, the method is fantastic for dating human remains. The problem is that beyond about 40,000 years ago, the estimates of C-14:C-12 ratios are not terribly accurate, since most of the C-14 has already decayed. After 5,700 years, only half of the C-14 originally incorporated into the tissue when the organism was alive is still there, and after 11,400 years only a quarter is still present. By the time we get to 40,000 years, only one sixty-fourth of the original C-14 is still present – less than 2 per cent. This makes the sample extremely susceptible to contamination by minute quantities of modern material, which would have the effect of making the dates appear to be more recent than they actually are. For this reason, radiocarbon dating tends to be most useful for remains that are younger than around 30,000 years, and it is the method of choice for archaeological sites of the past 10,000 years, where it is extremely accurate.

Once we get beyond 40,000 years, though, we have to use isotopes that decay at a slower rate. Two of these are Potassium-40 (K-40) and Uranium-238, which have half-lives of 1.25 billion and 4 billion years respectively. The problem with the more stable isotopes is that they are not usually found in the stones and bones themselves, and therefore they can be applied only to the sediments surrounding the remains – typically volcanic ash in the case of the former and lake sediments in the case of the latter. Thus, you have to have been very lucky with your choice of sites to be able to use them. Thankfully, the geological activity of Africa’s Rift Valley has meant that K-40 dating can be widely applied there.

But what if you aren’t so lucky? In particular, what if your remains are beyond the useful limit of radiocarbon dating, but they aren’t found in sediments that allow you to use the other methods? Then we have to rely on a collection of three relatively new techniques in the isotopic arsenal called – rather intimidatingly – thermoluminescence, optically stimulated luminescence and electron spin resonance. All rely on the observation that naturally occurring radiation causes electrons – another type of subatomic particle – to accumulate in small crystalline
defects in a substance at a steady rate, depending on the level of exposure to an ‘electron-cleansing’ radiation source such as fire or sunlight. There are many assumptions about the degree to which electrons had accumulated in the defects, known as traps, before being exposed to the cleansing radiation source. Also, there are assumptions about the variability in radiation exposure over time. For these reasons the dates obtained using luminescence and resonance methods are not as accurate as those obtained with C-14 or K-40 dating. However, for many sites they are the only option available.

It is exactly these last techniques which have been most widely applied in Australia. In particular, several objects obviously manufactured by humans – some of them associated with artistic depictions on rock faces – have been dated to more than 40,000 years ago. Of course, with the uncertainty of the techniques, it is difficult to know how accurate these dates are. But there is evidence from other sources that humans have been in Australia for a very long time indeed. Richard Roberts and his colleagues at the Australian National University, investigating the relatively unsophisticated tools used by these early people, have inferred dates as great as 60,000 years ago for one site in the Northern Territory.

The weight of palaeoanthropological evidence is now clearly in favour of a very early settlement of Australia by modern humans – perhaps as early as 60,000 years ago. But the earliest archaeological sites on the south-east Asian mainland date to less than 40,000 years ago. How could humans have been in Australia 20,000 years before this – surely they came from south-east Asia? The answer to this conundrum will take us back to Africa, where we need to pay a visit to the Garden of Eden.

Africa is the most equatorial continent on earth. The entirety of its landmass is found between latitudes of 38°N and 34°S, and 85 per cent of its land area is in the tropical zone between Cancer and Capricorn. Sea-level freezing temperatures are rare in Africa – uniquely among all the continents. While the interior deserts of the Sahara and
the high volcanic mountains of east Africa are inhospitable to humans, most of the continent is surprisingly benign. Africa contains the Old World’s largest uninterrupted tract of rainforest, and the savannahs of the east and south support a huge variety of large mammals. The combination of rainforest and savannah in close proximity, again unique in the Old World, is probably part of the reason that humans evolved there. Hominid bipedalism was almost certainly an early adaptation to the treeless grasslands of Africa, perhaps as early as 5 million years ago, where more resources could be exploited by leaving the aerial safety of the deep forests.

Africa was not always in the location it occupies today. Through the vagaries of plate tectonics, it spent most of its time between 200 and 20 million years ago migrating around the southern Indian Ocean, eventually bumping into the Eurasian landmass around 15 million years ago. It was at this time that the great apes began to disperse around the world as part of the first ‘African Exodus’. Those that went east evolved into the orang-utan and gibbon – the species favoured by Eugene Dubois as our most likely ancestors. The apes that stayed evolved into the chimpanzee and gorilla – and eventually, perhaps 100,000 to 200,000 years ago, into anatomically modern humans. During this entire sequence, Africa remained in the same position geographically. But, as with the other continents, the climate has fluctuated dramatically in the past few hundred thousand years.

The field of palaeoclimatology investigates the climate of bygone eras. The earth at 150,000 years ago was nearing the end of what is known as the Riss glaciation. On average, the temperatures were 10°C colder than they are today, although there was substantial variation among the continents. Around 130,000 years ago it started to warm up, and tropical Africa began to get more rain as the sea levels rose and moisture re-entered the atmosphere. A period of gradual cooling began around 120,000 years ago, accelerating after 70,000 years ago. This pattern would continue (with short-term fluctuations) for the next 50,000 years, reaching its nadir around 20,000 years ago.