Origins

by Lewis Dartnell

  The primeval trees of the Carboniferous Period, which would provide the vast reserves of coal fuelling the Industrial Revolution, and still give us a third of the energy we consume today,60 were a kind of plant known as spore-forming. Like ferns today, they reproduced by releasing spores on the wind which, if they fell on accommodating ground, germinated and grew into a tiny, green, leafy plant in their own right, but with only one-half of the full set of genetic material. It is this separate plant stage that had the equipment for sex, and they produced sperm that swam through films of water in the soil to an egg cell on a nearby plant. Once fertilised to reconstitute a complete double set of chromosomes, the egg then grew into a new full-sized tree. This seems a truly bizarre way of reproducing. It is as if humans procreated by spraying their sperm and eggs onto the ground in front of them, which each developed into a miniature version of themselves, and which then had to mate with each other to create an adult person. Moreover, this reproductive strategy worked fine for the spore-forming plants in the swampy basins of the Carboniferous, but they were biologically restricted to soggy soils by this alternating life cycle.

  Gymnosperms – plants with ‘naked seeds’ – emerged at the end of the Carboniferous and developed into all the evergreen conifers familiar to us today, including fir, pine, cedar, spruce, yew and redwood. They evolved to effectively suppress that intermediate phase of the life cycle. Once pollinated, gymnosperms produce seeds that are exposed on the scales of their cones. The seeds fall to the ground, safe within their protective casing and containing a small provision of stored energy, and wait for the right conditions to sprout. This evolutionary innovation released plants from the wetlands. (In some ways it’s analogous to the evolution of reptiles which, unlike amphibians, didn’t need to return to the water to reproduce.) As the gymnosperms spread around the world other plant species became either literally over-shadowed – bracken and other ferns mostly survived in the gloomy understorey within forests – or, like ginkgo in central China, continued to thrive only in isolated pockets. Gymnosperms are still very common today, growing as the dense conifer forests of spruce, pine and larch in the taiga ecosystem that stretches between the arctic tundra and the grasslands of the North American prairies and Eurasian steppes. They have been important through human history as sources of softwood for construction timber or pulp for paper, and feature as a minor part of our diet, for example in the form of pine nuts toasted and tossed into a salad or ground into pesto.

  The naked-seed gymnosperms ruled the Earth’s vegetation for around 160 million years, but it is angiosperms that dominate the plant world today, both in terms of their rich diversity of species and the range of different habitats around the planet they’ve come to master: deciduous woodlands in temperate regions, tropical rainforests, vast plains of grass across drier regions, and cacti in deserts. Angiosperms have taken their sex lives to an even higher level of refinement. Their eggs are not left naked but are contained in a special organ, originally adapted from a curled-up leaf, within which the seeds then develop – angiosperm means ‘encased seeds’.61

  A far more noticeable defining feature of the angiosperms, however, is the way they adorn and advertise their sex organs with flamboyant displays in the development of the flower. This evolutionary invention enabled angiosperms to recruit a huge range of insects – as well as birds, and some bats and other mammals – to help them transfer pollen from one plant to another.62 The first flowers were probably simply white, but as these plants and their pollinators developed together – one of the greatest stories of co-evolution in the history of life on Earth – the world exploded in a profusion of floral colours and heady scents. The specialised sex organs of flowering angiosperms not only allowed them to co-opt animals into helping them with their reproduction, but the ovary containing the seeds also developed into fleshy means to help them disperse: it produced fruit.

  By the late Cretaceous, the last period of the dinosaurs, the plant world on our planet would have already begun to look pretty similar to today, with sycamore, plane, oak, birch and alder tree families well established. But there was one glaring exception. The open, unforested plains in the drier areas of the continents would still have seemed eerily different. Although early forms of heather and nettles existed, grass species didn’t evolve until the end of this period.63 The dinosaurs roamed over terrain entirely devoid of grass.

  Our evolution as primates and our development as hunter-gatherers depended on the fruit, tubers and leaves of angiosperm plants. And the agriculture we adopted is also almost entirely reliant on angiosperms. Cereals are angiosperms: in fact, the grain we harvest is botanically the fruit of the grass plant.64 Signs of grass first appear in the fossil record from about 55 million years ago, but with the persistent cooling and drying of the planet through the Cenozoic era, grass-dominated ecosystems became established in many parts of the world between 20 and 10 million years ago.65 So not only was our own evolution driven by the aridification of East Africa, but the cooling and drying of the world as a whole created the conditions for the spread of the plants we would come to domesticate as staple crops for feeding our civilisations through history. And virtually every other plant we eat is also a member of one of eight different families of angiosperm.

  After the grasses, the second most important family are the legumes, which include peas and beans, soya beans and chickpeas, as well as the alfalfa and the clover we feed to our livestock. The brassicas include rapeseed and turnip, and a single species of this family, a weedy mustard plant, was transformed by accentuating different features of the plant through selective breeding to give us cabbage, kale, Brussels sprouts, cauliflower, broccoli, and kohlrabi.66 Other angiosperm groups include the nightshade family of potatoes, peppers and tomatoes; the family of gourds, pumpkins and melons; and the parsley family that also includes parsnip, carrot and celery.

  Most of the fruit we consume comes from either the rose family (such as apples, pears, peaches, plums, cherries and strawberries) or the citrus family (oranges, lemons, grapefruit, kumquat). The family of palm trees have also played an important role in history, giving us the coconut and, more influentially, the date, which served as a light and concentrated food source for the trade caravans crossing the deserts of the Middle East.

  Across these families of angiosperms we eat different parts of the plant. We cherish fruit which were evolutionarily designed by angiosperms to be attractive and tasty to animals to help them spread their seeds. Plants also create internal energy stores to power their growth the following spring and these are the root and stem vegetables we cultivate. Swollen roots include cassava, turnips, carrots, swedes, beets and radishes, and the tuber of a potato or yam is the swollen section of the plant stem. We eat the leaves of cabbage, spinach, chard and pak choi, as well as other salad plants and herbs; and the cauliflower and broccoli we consume are in fact immature flower heads. So overall, not only do we feed ourselves on grass, but also relatives of the rose bush and deadly nightshade. And beyond providing food angiosperms also give us fibres, such as cotton, flax, sisal and hemp, and a range of natural medicines.

  THE CIVILISATION APP

  While we cultivate and munch our way through a pretty broad range of different kinds of angiosperm plant, we have been far more limited in the sorts of large animals that we domesticated: we’ve selected them exclusively from just two categories of mammal.

  The first true mammals emerged around 150 million years ago, but it was the mass extinction of species 66 million years ago, wiping out the dinosaurs, that allowed our mammalian ancestors to spread into the niches now left vacant by reptiles. The three major orders of mammals that dominate the world today, however, did not emerge and begin diversifying until 10 million years later. These are the artiodactyls, perissodactyls and primates – collectively known as APP mammals.67 fn5

  We ourselves belong to the primates, as we saw in Chapter 1, and so they require no further introduction. Artiodactyls and perissodactyls, on the oth
er hand, may sound like alien species, but you are intimately familiar with them. In fact, you could argue that they provided the very basis for human civilisation. They are the two branches of ungulates, or hoofed mammals. The artiodactyls are the even-toed or cloven-hoofed ungulates; the perissodactyls are the odd-toed ungulates.

  The even-toed artiodactyls include pigs and camels, as well as all the ruminants: antelope, deer, giraffe, cow, goat and sheep. Ruminants deal with the challenge of breaking down tough grass by regurgitating the cud to chew it again, and then use bacteria in the first of four compartments in their stomachs, the rumen, to ferment the plant material and help break it down chemically, before it is passed through the rest of the digestive system to absorb the nutrients. (As we saw earlier, humanity found technological solutions to the same biological problems.) Artiodactyls are the dominant large herbivorous animals in the world today. Their cloven hoof is made up of two toes, which correspond to the third and fourth fingers on your hand.fn6

  The odd-toed perissodactyls include horses, donkeys and zebras, as well as tapirs and rhinos. Perissodactyls have either three toes like the rhinoceros or just one like the horse. In effect, horses gallop around on the same finger you would use to flip someone the bird. In contrast to ruminants, they are hind-gut fermenters with a simpler stomach. They host bacteria to ferment and help release the nutrients from the vegetation in a greatly enlarged pouch in their intestine, called the cecum.fn7

  It is astonishing that the vast majority of the large animals that we domesticated over the last 10,000 years, and which human civilisation came to depend upon for their meat, secondary products and muscle power, are all members of just one group of mammals. But there’s something equally fascinating and profound about how these ungulates first emerged.

  A FEVER OF THE WORLD

  The surprising fact is that the artiodactyl and perissodactyl orders, along with the primates, all emerged suddenly within a period of about 10,000 years, in a burst of evolutionary diversification that occurred 55.5 million years ago. It turns out that both our ancestors who would eventually evolve into Homo sapiens in East Africa and the groups of animals which became so vital for domestication and the development of civilisations, all appeared in the same blink of planetary time. And the event that seems to have triggered the rapid emergence of these crucial APP mammals was a singular planetary spasm – an extreme spike in the world’s temperature.70 fn8

  This exceedingly rapid heating of the world’s climate marks the boundary between the geological epochs of the Palaeocene and the Eocene, and so is known as the Palaeocene–Eocene Thermal Maximum – PETM for short. Over a very brief geological span of less than 10,000 years, massive amounts of carbon (carbon dioxide, CO2, or methane, CH4) were injected into the atmosphere, creating a powerful greenhouse effect, and the global temperatures jumped rapidly by 5–8 °C in response.71 This temperature spike made the world the hottest it has been for the past few hundred million years.72

  Despite this huge jolt to the environment, no mass extinction on the scale of the end-Cretaceous or end-Permian (see here, here) was triggered, although the ecosystems of the world were utterly transformed. Tropical conditions extended all the way to the poles, with broad-leafed trees, crocodiles and frogs all thriving within the Arctic.73 The PETM caused the disappearance of some deep-sea amoeba, called foraminifera, that were unable to cope with the warmer waters and reduced oxygen at depths,74 whereas plankton such as dinoflagellates bloomed in the balmy sunlit surface of the oceans. The global environmental disruption of the PETM also drove rapid evolution in many animals,75 and in particular this temperature spike seems to have ushered the emergence of the new APP orders of mammals.76

  We would expect that the rapid heating of Earth’s atmosphere was the result of volcanic activity, as happened on numerous occasions in our planet’s history. But the curious thing is that the cause of most of this enormous and sudden release of carbon that triggered the temperature spike wasn’t volcanic – it was biological.fn9

  It is thought that an initial volcanic eruption released enough carbon dioxide to warm the oceans sufficiently to destabilise underwater deposits of a kind of ice called methane clathrate. Clathrate ice forms under the cold and high-pressure conditions of the sea floor and traps methane gas originally produced by decomposition bacteria. But if these clathrates are warmed they break down and release the trapped methane to bubble up through the water and into the atmosphere. Methane is one of the most powerful greenhouse gasses – its heat-trapping effect is over 80 times stronger than carbon dioxide – so the first methane that was unleashed caused further warming which destabilised even more clathrate ice in a diabolical feedback process. Alongside the clathrate ice, more greenhouse gas probably belched out when the permafrost in Antarctica began to thaw and wildfires became more frequent in the warming climate.77 The initial volcanic eruption was like the detonator that set off the main explosive charge of biological carbon release, resulting in the sweltering climate of the PETM.

  Although severe, the temperature spike was very brief in geological terms: the atmosphere and the global climate returned to their earlier levels again within 200,000 years or so.78 Yet this global warming – a short, intense fever of the world triggered by a great methane flatulence of the oceans – led to the emergence of the three orders of mammals most fundamental for all human history. Artiodactyls, perissodactyls and our own group the primates all appeared suddenly right at the beginning of the PETM, and then rapidly dispersed across Asia, Europe and North America.79

  If this extreme temperature blip drove the emergence of the APP orders, it was the global cooling and drying over the last few tens of millions of years that created the ecosystems the artiodactyls and perissodactyls came to dominate. As the grasslands spread around the desiccating continents, the herbivorous ungulates followed and diversified into a large number of different species, including the ancestors of our cows, sheep and horses. So the grasslands that supplied the cereal crops we came to cultivate also provided the evolutionary theatre for the emergence of the large ungulate animal species that we domesticated. But when the world emerged from the last ice age, and human communities around the globe began to settle down and domesticate the wildlife they found around them, both cereal and ungulate species were not evenly distributed around the planet. And this had profound implications for the subsequent course of civilisations.

  THE EURASIAN ADVANTAGE

  Of the roughly 200,000 plant species in the natural world, only a couple of thousand are suitable for human consumption, and just a few hundred of these offer potential for domestication and cultivation. As we saw earlier, the staples that have supported civilisations across the planet throughout history are cereal crops, but the wild grass species that these cereals were domesticated from were not uniformly spread around the world. Of the fifty-six grasses offering the largest, most nutritious seeds, thirty-two grow wild in south-west Asia and around the Mediterranean, six are found in East Asia, four in sub-Saharan Africa, five in Central America, four in North America, and only two each in South America and Australia.80

  Thus from the very beginnings of agriculture and civilisation, Eurasia was richly endowed with wild grass species amenable to domestication by humanity and suitable for supporting growing populations. And not only was Eurasia by chance blessed with this biological bounty, but the very orientation of the continent greatly promoted the spread of crops between distant regions. When the supercontinent Pangea fragmented, it was torn apart along rifts that just so happened to leave Eurasia as a broad landmass running in an east–west direction – the entire continent stretches more than a third of the way around the world, but mostly within a relatively narrow range of latitudes. As it is the latitude on the Earth that largely determines the climate regime and length of the growing season, crops domesticated in one part of Eurasia can be transplanted across the continent with only minimal need for adaptation to the new locale. Thus wheat cultivation spread readily from the up
lands of Turkey throughout Mesopotamia, Europe and all the way round to India, for example. The twin continents of the Americas, by contrast, though joined by the bridge of the Panama Isthmus, lie in a north–south orientation. Here, the spreading of crops originally domesticated in one region to another entailed a much harder process of re-adapting the plant species to different growing conditions. This fundamental distinction in the layout of the Old World versus the New, itself born from plate tectonics and the aimless wandering of the continents into their current configuration, gave the civilisations of Eurasia a great developmental advantage through history.81

  The distribution of large animals around the world was equally uneven, and here societies across Eurasia received another advantage. The attributes of a wild animal that make it amenable to domestication by humans include offering nutritious food, a docile nature and lack of inherent fear of humans, a natural herding behaviour, and the ability to be bred in captivity. Yet only a relatively small number of wild animals qualify on all these factors.82 Of the 148 species of large mammals around the world (heavier than 40 kilogrammes), 72 are found in Eurasia, of which 13 were domesticated. Of the 24 found within the Americas, only the llama (and its close relative the alpaca) was domesticated in South America. North America, sub-Saharan Africa and Australia completely lacked domesticable large animals. The five most important animals through human history – the sheep, goat, pig, cow and horse – as well as the donkey and the camel that provided transport in particular regions, were present only in Eurasia, and within a few thousand years of their domestication had spread across the continent.83 It is the large mammalian species that have proved most influential throughout history, not only for their meat, but also for their secondary products (milk, hide and wool), and their muscle power.