The Incredible Human Journey

by Alice Roberts

  In the 1990s, John Johnson was part of a team of archaeologists and palaeontologists who, following up Phil Orr’s work, undertook a comprehensive survey of the island, looking for Pleistocene fauna in particular. They didn’t find any more human bones, but they did discover plenty of mammoth fossils, all around the coast of Santa Rosa, in alluvial sediments cut through by water action – just like the bank with the mammoth fossils that John had shown me. They also uncovered an almost complete skeleton of a pygmy mammoth, and when they radiocarbon dated the bones they found them to be almost exactly the same age as Arlington Woman. In fact, there seemed to be an overlap of around two hundred years. This was an important result. Mammoths had been on the island for around 47,000 years, and now the archaeologists could be sure that there was a period when pygmy mammoths and humans were contemporary on Santa Rosa, as Orr had suspected. Orr also believed that humans had played a key role in the extinction of these diminutive mammoths. It is difficult to support such a theory on the basis of the Santa Rosa material alone, founded on dates from ‘one mammoth, one human, on one island’,3 and there is no ‘smoking spear’ to prove human predation on mammoths. Nevertheless, it still seems a remarkable coincidence that, within two hundred years of humans arriving on Santa Rosa, the mammoths had disappeared. In many ways, the evidence from this island is tantalising but frustrating. In spite of some continued archaeological investigations on the island, the only evidence of humans there in the Pleistocene is those thigh bones. Nothing else: no other bones, no evidence of any occupation sites. What was that woman doing on Santa Rosa? If she was part of a band of hunter-gatherers living there, no other trace of these people has appeared – yet. Humans may have killed off the miniature mammoths on the island, but for more evidence of human involvement in the extinction of Pleistocene fauna, and of the beasts themselves, I would need to return to the mainland.

  John and I had wandered down Arlington Springs gully all the way to the seashore, and we sat down on a sand dune to talk about his other research, in genetics and phylogeography. John had become fascinated by the language families of the Californian Native Americans, including the Chumash.

  Languages can certainly suggest population origins and movements, but they are also transferable – like other aspects of culture – and so it’s very difficult, even impossible, to reconstruct the deep evolutionary history of people around the world on the basis of variation in language. For John, the suggestions emerging from the linguistic studies warranted further investigation, and mtDNA analysis seemed to be the perfect tool. But it was a telephone call from a friend that had really set John off on the trail of tracing ancient migrations.

  ‘A few years ago, a friend of mine called me up and told me there was this tooth from an ancient jaw from southern Alaska, that had DNA that was a very rare type.’

  I asked him how old the tooth was.

  ‘It was 10,300 years old – the oldest skeletal remains in the Americas from which they’ve successfully obtained DNA.

  ‘But the remarkable thing is that this rare type of DNA – which is only found in 2 per cent of Native Americans overall – matched 20 per cent of the samples I had taken from Chumash Indians, here in California.

  ‘And when we looked at a wider database of all Native Americans, we found that this type was also in southern Alaska, in north-west Mexico, in coastal Ecuador, in southern Chile, in southern Patagonia, and in prehistoric burials in Tierra del Fuego. All the way down the Pacific coast.’

  ‘So do you think this is evidence for a coastal dispersal?’ I asked.

  ‘I think it is the best evidence to date for an ancient coastal migration: it suggests that this particular group gradually moved down the Pacific coast, taking advantage of coastal resources, and left behind descendants who still live in all of these areas.’

  John and his colleagues had looked at samples from 584 Native Americans, from a geographical perspective. Haplogroup A was more common along the coast, but there was also a rare subgroup of haplogroup D that seemed to echo a migration into the Americas all the way down the Pacific coast of North and South America.5, 6 This was the same rare type that had been found in DNA extracted from the ancient tooth discovered on Prince of Wales Island, in Alaska.7 A recent study of Native American autosomal DNA markers – within chromosomes in the nucleus – also revealed a pattern, not only of decreasing diversity from north to south, indicating colonisation in that direction, but also suggesting that the coastal route was important.8 Just possibly Arlington Woman may have been a descendant of the first ocean-going, beachcombing Americans.

  Hunting American Megafauna: La Brea Tar Pits, Los Angeles

  Having had my appetite whetted by the remains of those diminutive mammoths on Santa Rosa, I wanted to find out more about the large animals that roamed the Americas during the Ice Age.

  Soon after I started working at Bristol University, I met colleagues in Earth Sciences who showed me some of the collections held in that department. There was one room in particular that fascinated me: there were samples of rocks from around the world, and, in a large glass cabinet, the impressive skeleton of a sabre-toothed cat. The bones were dark brown, almost as though they had been carved out of ebony. The stuff that had turned the skeleton that colour was tar. The cat was a composite skeleton, made from bones that had been dug up from the La Brea tar pits in Los Angeles. It had been brought to Bristol University by the explorer and famous palaeontologist Bob Savage, who was a professor in the Department of Earth Sciences during the fifties and sixties.

  I was, therefore, very excited to be visiting the tar pits themselves. I had driven down the Californian coast from Santa Barbara to Los Angeles (stopping off to watch surfers at Malibu Beach on the way), and now I was heading into the heart of the city, to get a close look at the amazing palaeontological treasures that had emerged from the tar. I approached La Brea Museum around the edge of a lake, filled mostly with water, but with tarry edges, and huge methane bubbles rising to burst on the surface every few seconds. There I found myself face to face with life-size – full-size – mammoths. A great bull mammoth had become mired in the tar pit, and, on the bank, a cow and her calf were looking on helplessly. They were good models: it was almost as if I’d been transported back 20,000 years in time.

  Inside the museum itself were reconstructed skeletons of a range of animals that had met sticky ends – in the most literal sense – in the tar pits: giant sloths, horses, camels, mammoths and mastodons, sabre-toothed cats, lions and dire wolves, an extinct species of wolf that used to live in both North and South America. In a glass-walled laboratory, palaeontologists worked away, cleaning and preparing the specimens that were still emerging from the tar pits. I went through a door that took me from the public side of the museum into laboratories and store rooms, where I met the curator, John Harris. I quickly discovered that John had also been at Bristol University, studying for his Ph.D., and he remembered Bob Savage’s sabre-toothed cat from La Brea.

  John showed me some of the museum stores: they comprised what seemed like endless corridors lined with shelves and trays containing thousands – millions – of bones from the tar pits. The bones were not fossilised in the true sense of the word: they had not turned to stone but were still bone because the tar had preserved them. Then John took me outside, to a pit where excavation was still going on. The pit was almost 10m deep and about 10m square. Down at the bottom of the pit, working away in the black, sticky sludge, were three young palaeontologists – Andrea Thomer, Michelle Tabencki and Ryan Long – all of whom were themselves well covered in tar. I sensibly donned a forensic-like white suit and started to descend the ladder into the pit, where I could safely stand on boards to one side.

  ‘This is a strange job,’ I offered, gingerly making my way down the ladder into the tarry trench.

  ‘It’s basically the strangest job in the world … digging up the bones of dead animals out of tar,’ said Andrea.

  ‘And it doesn’t look like very nice s
tuff to excavate.’

  ‘No, it’s terribly sticky. Especially in the summertime. As it gets hotter it gets stickier.’

  Around her Wellington-booted feet I could see bones sticking up out of the tar. The more I looked, the more I saw. It was like a mass grave of Ice Age beasts.

  ‘We’ve had visiting palaeontologists come by who are just astounded to see this many fossils in one place. It’s probably one of the richest fossil deposits in the world,’ Andrea told me.

  ‘And what type of species have you found?’ I asked her.

  ‘Well, the most common animal we find is the dire wolf. And the second most common is the sabre-cat. We have a very strange predator– prey ratio – we have way more predators than we do animals that they eat. We think it’s because of the way that the animals became trapped in these tar pits: a large herbivore would walk in, get stuck, and then all of the carnivores would come and try to eat him, and then they would get stuck. About 70 per cent of the animals are predators rather than prey, and that’s completely backwards.’

  ‘And how old are these fossils that you’re getting out of this pit?’

  ‘Well, down at the bottom, they’re 40,000 years old. And they’re not actually fossilised. They’re still bone – you get incredible detail.’

  Michelle was trowelling the surface of the tar, removing a wet, oily layer so that excavation would be easier.

  ‘It’s called glopping,’ she said. ‘We have to do this every day before we really start work excavating bones from the pit. The tar is constantly rising up. If we didn’t do this every single day, the pit would start filling up with liquid asphalt again. We’re fighting nature, and it’s a losing battle.’

  Climbing back up out of the pit, I asked John about the scale of the operation.

  ‘Since 1969 we’ve taken more than 70,000 bones out of this particular locality. All together we’ve got something like three and a half million specimens in our collections – representing more than 650 species of animals and plants.’

  I asked him if they excavated all year round.

  ‘No. We now only excavate during the summer. We do a ten-week excavation season and in just ten weeks we take out between one and two thousand bones.’

  It was clearly a huge job and the tar pits seemed almost bottomless. And it wasn’t just the planned excavations at La Brea itself that kept the palaeontologists busy. Construction work in downtown LA often turned up tarry surprises, and a recent sewer trench had unearthed tonnes of tar-soaked sediment – full of bones. I looked over to where John was pointing, just behind the tar pit where excavations were ongoing, and there were masses of industrial containers, full to the brim of sediment, waiting to be excavated. John was going to have to pull his workers out of the pit so that they could focus their attention on this unexpected haul of tarry bones.

  Among all those Ice Age skeletons, only one human has ever been found.

  ‘It’s not that surprising,’ said John. ‘If you get stuck in a tar pit then you’re likely to meet your end there – unless you have friends to pull you out.’

  The tar pits gave a wonderful insight into the richness of animal life in ancient California. It was also obvious that the vast majority of these large animals were no longer with us. Just as in Eurasia and Australia, the arrival of humans in the Americas seemed to coincide with the demise of the megafauna.

  ‘The peculiar thing,’ John continued, ‘is that about 13,000 years ago these large creatures disappeared – and it’s one of the big mysteries of American palaeontology.

  ‘Several people have come up with different ideas about how and why they disappeared. This was, after all, right at the end of the Ice Age; there would have been climatic change and environmental change. But that had happened at least ten times during the Pleistocene so that, in itself, climate change wouldn’t have caused the extinction of the megafauna.

  ‘Thirteen thousand years ago is about the time when humans arrived in North America, and some people think that it was humans hunting the megafauna, or introducing diseases against which the megafauna had no immunity, that was part of the cause.’

  La Brea tar pits don’t contain any evidence of human predation, but there are other sites in America that show, unequivocally, that humans were hunting mammoths. But, just as on other continents, whether or not humans hunted the mammoths and other megafauna to extinction, or whether changing climate and environment played a more significant role, it is difficult to know.

  But John said that there was another, rather intriguing, theory about the disappearance of the American megafauna that had recently exploded on to the scene.

  ‘The third suggestion that’s come up recently is that, apparently, there was an explosion of an asteroid right over the Great Lakes region of North America at around 13,000 years ago …’

  When I was in Arlington Springs and looking at the side of the gully (now eroded much further back than when Phil Orr discovered Arlington Woman), John Johnson had pointed out several black layers in the section. Most of them were probably due to forest fires that occasionally ripped through the island’s vegetation. I had seen the results of recent forest fires in the hills above Santa Barbara: blackened trees and a thick layer of ash over the ground.

  But it seems that one of these black layers represents something more than random and sporadic forest fires. Across North America, at more than fifty sites, a particular black layer has been found that dates to 12,900 years ago. The timing of this layer coincides with the onset of the Younger Dryas cool period, and with the extinction of the North American megafauna: mammoths, mastodons, ground sloths, horses and camels. Proponents of the asteroid hypothesis suggest that human overkill and climatic cooling are both inadequate explanations of megafaunal extinction. While mammoth and mastodon kill sites have been identified, there are none for the other thirty-three genera of megafauna that also disappeared. And previous cold spells similar in degree to the Younger Dryas had not resulted in mass extinctions.1

  Geological analysis of the 12,900-year-old black layer from several sites revealed that it contained not only charcoal, carbon spherules and glasslike carbon indicative of intense forest fires, but also rather more strange components, like nanodiamonds and fullerenes containing extraterrestrial helium. These are very rare things indeed: they are found in meteorites and in the ground at extraterrestrial impact sites. So the suggestion is that the continent-wide wildfires were caused by a comet hit.1

  No crater has yet been identified, but the researchers suggest that perhaps there is none to find: maybe the comet crashed into the Laurentide ice sheet, making very little mark on the earth beneath the ice – or perhaps it exploded in the air. In 1908, something from space – either a burnt-out comet or an asteroid, less than 150 metres in diameter, exploded in the atmosphere above Tunguska in Siberia. The resulting airburst set fire to 200km2 of forest, and knocked down trees over a wider 2000km2 , while leaving no crater. Perhaps the widespread fires of 12,900 years ago were caused by multiple airbursts or impacts.

  Maybe this proposed impact even set off the Younger Dryas, with clouds of soot and smoke from the forest fires blocking out sunlight. Perhaps the impact knocked off parts of the ice sheet, with rafting icebergs bringing down the surface temperature of the Arctic and North Atlantic oceans. It’s an intriguing hypothesis, and the timing certainly works. An extraterrestrial impact around 12,900 years ago, sparking off continent-wide forest fires, environmental destruction, and subsequent cooling could have wiped out the American megafauna.

  But other archaeologists argue that there is no need to look to the stars for an explanation of the disappearance of the large Pleistocene mammals of North America. For Gary Haynes of the University of Nevada the disappearance of thirty-three genera of large mammals at the same time as the appearance of distinctive Clovis spear points in the archaeological record is not a coincidence.2 He argues that humans have been known to hunt herd animals to extinction without any ‘help’ from climate change or ex
traterrestrial impacts. Haynes also draws attention to the knock-on environmental consequences of losing large ‘keystone’ species, like mammoth and mastodon. These animals modify their environments, opening up areas of grazing for smaller herbivores. In Africa, large herbivores help to keep grassland savannah productive. Archaeologists have argued that the loss of these large grazers in Beringia may even have caused a shift from productive grassy steppe to moss-tundra. In other words, once humans had impacted particular species of megafauna, the loss of those animals would have further ecological consequences, spreading out to affect other species as well.2

  Haynes argues that the climatic changes suggested by some to be instrumental in the demise of the megafauna did not happen at the right time, as the Younger Dryas cold spell succeeded rather than preceded some extinctions (although this is not necessarily at odds with the comet hypothesis, as the more immediate effects of the impact could have finished off the megafauna). And there are the archaeological kill sites that demonstrate that the Clovis hunters despatched mammoths. Haynes makes the case that mammoth and mastodon hunting would have been a sensible strategy for the Pleistocene hunters of North America.

  The idea of Clovis people as ‘big-game hunters’, descendants of the Eurasian Upper Palaeolithic hunters of the steppe, has been around a long time. This image is very persuasive: it permeates our culture, fostered by artistic reconstructions and in film. The iconic image of plucky Ice Age hunters clad in furs, bravely bringing down a mammoth, is familiar to us all. But is it an accurate representation?

  Clovis Culture: Gault, Texas