by Tim Flannery
The six bear species living today (polar, brown, Asiatic black, American black, sloth and sun) have evolved from a common ancestor over the last five million years. While they remain very different in appearance and ecology, DNA analysis reveals an astonishing degree of hybridisation in their lineage. For example, polar bears have hybridised with brown bears, so that 8.8 per cent of the brown bear genome comes from polar bears. This means that ‘Pizzlies’ (as recent polar bear/brown bear hybrids are known), are not a new phenomenon, but have been produced for hundreds of thousands of years. Among many other crossings noted in the study, brown bears have hybridised with American black bears, and Asiatic black bears with sloth bears, and sloth bears with sun bears. The researchers concluded that hybridisation between various bear species has been going on for millions of years, so that when the crosses between bear species are included on the bear family tree, the diagram looks more like a family network.4
The history of hybridisation among elephants is, if anything, even more astonishing. A recent study by the Harvard-based palaeogeneticist Eleftheria Palkopoulou and her colleagues, which includes the three living species (African, African forest and Asiatic) and three extinct kinds (European straight-tusked, woolly mammoth and American mastodon), revealed that elephants have hybridised throughout most of their history. Indeed, some extinct elephants result from such extensive hybridisation that they are not easily classifiable in the Linnaean system.
In summarising their findings, Palkopoulou’s team say: ‘The capacity for hybridisation is the norm rather than the exception in many mammalian species over a time scale of millions of years.’5 They also speculate that the sharing of genes through hybridisation might have helped species migrate and adapt to threats and opportunities by allowing species to acquire genes from near relatives. Looked at this way, we can think of species, such as our own, that have now lost the ability to hybridise because our close relatives are extinct, as vulnerable and isolated.
Were hybridisation extensive enough, life would become one undifferentiated mass. So why do species exist? It turns out that there are mechanisms (known as species isolating mechanisms) that make the production of hybrids difficult. It is rare for an individual to overcome these barriers, but among the millions of individuals that comprise a species it is common for enough hybrids to be produced to allow genes to flow between species. Some species’ isolating mechanisms are behavioural—such as the possession of a particular mating call, which only females of a given species will respond to—or a preference for reproducing at a particular time of the year. Others, such as penis size or shape, are physical. But there are also genetic and epigenetic barriers. Sometimes genetic factors prevent a viable embryo from forming. But they can also result in most first-generation hybrid individuals being infertile, or having low fertility. In a phenomenon known as Haldane’s rule, this is particularly true for male hybrids among mammals. But if first-generation hybrids do manage to produce some offspring, the next generation often has improved fertility—though usually only with one or the other species (but not both) involved in the original cross. All of these barriers tend to limit, but not eliminate entirely, the flow of genes from one species to another.
Sometimes hybridisation does more than permit gene flow between species, instead creating an entirely new, hybrid species. Among the European species that arose by hybridisation is the European edible frog (Pelophylax kl esculentus); a widespread and economically important creature esteemed as a culinary delicacy in France. The parent species that gave rise to it—probably hundreds of thousands of years ago—are the European marsh frog and the European pool frog. As you might have noticed, the creature’s scientific name has a ‘kl’ inserted in it. This denotes that it is a ‘klepton’ or ‘gene thief’—a hybrid that requires another species to complete its reproductive cycle for it. Most kleptons are female, and some don’t use the genes of the male at all, merely deploying his sperm to stimulate the egg into development without fertilising it.6*
Even some mammal species arose via hybridisation. The golden jackal has recently been recognised as being two different species—a smaller one that originated with an early offshoot of the wolf lineage, and a larger kind that is closer to the modern Eurasian wolf and whose ancestors must have migrated into Africa before mixing with the smaller jackal and creating a new, hybrid species.**
The wisent—Europe’s largest surviving mammal—is a stable hybrid species that arose around 150,000 years ago, when aurochs and steppe bison underwent an extended period of hybridisation. Steppe bison (from which the American buffalo is descended) inhabited the mammoth steppe and vanished from Europe at the end of the last glacial period, while aurochs were creatures of the more temperate forested areas. Wisents carries mostly bison genes, with a healthy infusion (about 10 per cent) of aurochs genes, the mixed genetic heritage apparently helping the wisent survive changing conditions as the climate warmed and forests spread.7
Hybridisation in agriculture is different from hybridisation in nature, both because the conditions created by people allow hybridisation between species that would never hybridise naturally, and because we have selected for extreme traits in many domesticated forms. When domesticates become feral, or breed with non-domestic relatives, those interested in conservation face a quandary: should they seek to eliminate the hybrids, and so seek to ensure that the wild types are not overwhelmed by the domesticates? Some see the highly modified domestic creatures as a form of pollution—albeit genetic pollution—that because of the huge abundance of domesticates, can endanger their far rarer wild relatives.
For example, a case might be made that dog-wolf hybrids should be removed from nature for fear that dog genes might overrun the wolf population (an issue I shall return to). But a more difficult example involves the Scottish wild cat, in which the great majority of the population are hybrids between wild and domestic types. It might seem desirable to remove the hybrids, but to do so would leave a population so small as to be headed for extinction.
Hybrids present a particular problem when it comes to legal policy. Our major legal instruments for protecting species, including the Bern Convention and the US Endangered Species Act of 1973, deal with species, not hybrids. Indeed, the US Act has been described as ‘almost eugenic’ because it excludes hybrids from protection.8 Given our knowledge of the extent of hybridisation, this is problematic. And matters are made more difficult because defining hybrids is not always straightforward. A first-generation cross might stand out, but over time it becomes more and more difficult to detect hybrid animals. Indeed, most of our recent insights into the importance of hybridisation in nature come from DNA studies on animals that, at first glance, appear not to be hybrids.
Hybridisation can also result in heterosis—the scientific term for the production of ‘super fit’ hybrid individuals—many examples of which come from agriculture. Heterosis can be thought of as the opposite of inbreeding depression, the phenomenon whereby the offspring of individuals that are genetically too similar—for example, brother and sister—can suffer from debilitating maladies. Heterosis usually occurs when the parents are moderately different, for if individuals are too different, their genes often will not combine to form a viable embryo. Heterosis is well known to animal and plant breeders, who seek it out: grains that result from crossing different strains, for example, are often more disease resistant and grow faster.
An instructive example of a heterotic individual is ‘The Toast of Botswana’. The Toast is a cross between a female goat and a male sheep. As such, he is an exceedingly rare beast—goats and sheep being too different genetically to readily create a viable offspring. The Toast was born into the flock of Mr Kedikilwe Kedikilwe at the Botswanan Ministry of Agriculture, who noticed that the creature grew faster than the lambs and kids born at the same time. He was also astonished by the fact that it hardly ever got sick, even when an outbreak of foot and mouth disease afflicted the rest of the flock.
As its name sugge
sts, for a time after its birth The Toast was exemplary in every way. But when he reached puberty a problem arose: the creature became extremely libidinous, copulating with sheep and goats indiscriminately and even out of breeding season. This unseemly behaviour earned him the shameful nickname of Bemya, or ‘rapist’. Yet, despite his ceaseless efforts, The Toast fathered no young. Ashamed and annoyed at its fall from grace, Mr Kedikilwe had The Toast castrated.9
Hybrids are often noted for their libidinous ways—as if they understand that the only possibility for them to pass on their genes lies in indulging in many and varied couplings, in the hope that some way around the species isolating mechanisms can be found. But because we humans misapply moral standards to animals, we often cut short their efforts. Had Mr Kedikilwe stayed the knife, we may have learned a great deal more about heterosis and hybridisation.
Heterosis can affect far more than growth rates and disease resistance, for brain function and behaviour can also be influenced, as is evidenced by the mule. As Charles Darwin observed, the mule ‘always appears to me a most surprising animal. That a hybrid should possess more reason, memory, obstinacy, social affection, and powers of muscular endurance than either of its parents, seems to indicate that art has here outmastered nature’.10 We consider some of the mule’s key traits as observed by Darwin—reason, memory and social affection—as among our species’ most valued and distinctive characteristics: yet we never consider that they may result from heterosis.
Because of its position at the crossroads of the world, Europe has had many immigrant species that provided unprecedented opportunities for hybridisation. It may be this fact, as much as anything, that has driven evolution at such a rapid pace in Europe, and which in turn has lent many European species the capacity to colonise new and environmentally different lands. The pace of hybridisation in Europe has picked up substantially since the dawn of agriculture, and ever more hybrid species are being created. The Italian sparrow, for instance, is a hybrid between the Spanish sparrow and the house sparrow that originated in Italy sometime in the last 10,000 years.11 In Britain alone, at least six new species of plants have arisen through hybridisation since 1700, while hybrid super-slugs are becoming a plague in English gardens.12, As climate change brings evermore creatures to Europe, the rate of hybridisation is likely to skyrocket.
The idea that hybridisation may be ‘the norm’ among mammalian species for millions of years after they first arise—and that it may help them adapt to new conditions—is very challenging to many, and is diametrically opposed to the idea that they result from nature’s ‘grossest blunder’. But Fisher’s views of hybridisation are now as outmoded as his endorsement of eugenics. It’s now clear that species are not ‘fixed’ entities, but are permeable. Throughout European prehistory, immigration has created opportunities for heterosis to arise in the wild, and as a result European nature is all the better adapted. Perhaps in time we will come to value many hybrids, and to see that there can be no more dangerous concept than the idea of racial or genetic purity. At a minimum, our new understanding of hybrids means that a fundamental re-think of classification, endangered species legislation and laboratory-based gene transfer is overdue.
______________________
* The genetics of kleptons can be extremely complex, some eliminating the genes of one parent during the production of sperm or eggs. Three klepton hybrid species, all with the marsh frog as a parent, exist in Europe, and all have distinct genetic pathways for reproduction. In all three, the genes of the marsh frog are never lost.
** Neither of these species is closely related to the ‘true jackals’ of the genus Lupulella.
CHAPTER 24
Return of the Upright Apes
Between 5.7 million years ago, when a small ape strolled the seashore in what is today Cyprus, and 1.85 million years ago (when Homo erectus appears), there is no evidence of apes in Europe. Our lineage had been evolving in Africa, and the creatures that returned to Europe belonged to our own genus—Homo. Everything we know about them comes from a fossil site at Dmanisi, in Georgia, where a rich collection of Homo erectus, along with many other species, was found in the 1980s.1
Located on a promontory-like plateau overlooking the confluence of the Pinasauri and Masavera Rivers some 85 kilometres southwest of Georgia’s capital Tbilisi, the deposits are preserved under the medieval ruins of Dmanisi, which was taken from the Turks and rebuilt by the Georgian King David the Builder in the twelfth century. The bones are preserved in gullies that were cut into the plateau and subsequently became filled with sediments. In 1984 a team began a major excavation resulting in the discovery of an abundance of stone tools and hominid remains. Work at Dmanisi continues under the direction of David Lordkipanidze, Director of the Georgian National Museum, with new discoveries occurring every few years.
Dmanisi has forced a rethink of both human and European prehistory. The deposits are between 1.85 and 1.78 million years old, making the Homo erectus remains found there the oldest known.2 The brains of the Dmanisi Homo erectus are 600 to 775 cubic centimetres in volume (about half the size of the brains of anatomically modern humans). This is much smaller than those of other Homo erectus, and closer in size to Homo habilis (the African ancestor of H. erectus). One extreme view is that Homo erectus evolved in Europe from some earlier, as yet undetected species of Homo. Whatever the case, it is striking that, below the neck, the Dmanisi Homo erectus are similar to modern humans, though their arms retained some primitive characteristics typical of more arboreal ancestors.3 Another striking feature of the Dmanisi remains is their variability. Both large and very small individuals are included; palaeoanthropologists posit that if the five skulls recovered to date had been found in separate locations, they would be classified as belonging to several different species.
A toothless cranium of a male Homo erectus found in 2002 matched up perfectly with a toothless jaw found in 2003, and the discoveries open a window on the social life of the species. In many other animals, a lack of teeth means death: the individual starves. The toothless Homo erectus from Dmanisi provides the oldest evidence anywhere for the survival of such an impaired individual. Lordkipanidze argues that the man could only have survived with help; the Dmanisi Homo erectus must have been highly social, perhaps living in small family groups that cared for its less able members.4
Could Homo erectus speak? Rarely preserved parts of the cranium and spine unearthed at Dmanisi (including a string of six vertebrae) are shedding some light. The Dmanisi Homo erectus had the right respiratory apparatus to support speech—indeed it is within the range of our own species.5 And an enlarged depression on the inside of the cranium provides evidence that Broca’s area, the part of the brain that processes articulated language, was present, so it’s possible that language was used by the bipedal apes of Dmanisi.
Many palaeoanthropologists would reject the idea that Homo erectus had language, viewing the palaeontological data as akin to building a castle on sand. But we should be cautious; ever since Victorian gentlemen imagined the Neanderthals as lowly cave men and themselves as the acme of evolution, we have underestimated the capacities of our distant ancestors and relatives. And, with each new scientific finding, we’re discovering them to be more competent than we had previously thought.
The Homo erectus of Dmanisi were capable predators who repeatedly occupied the plateau for at least 80,000 years. It must have been a valuable strategic vantage point from which to watch for migrating animals. Fossilised hyena faeces and the bones of fourteen other carnivore species reveal that Homo erectus did not have sole tenure over the lookout. We can imagine a European Serengeti moving through the river valleys, with the predators descending from their lookout to kill, then transport the flesh to the hilltop to be eaten. The transported remains of elephants, rhinos, giant ostrich, extinct giraffes, seven species of antelope, goats, sheep, cattle, deer and horses have been found, the last two being especially abundant.6
Just how the various predators in
teracted can only be guessed at. It seems likely, however, that the giant hyena and Homo erectus, being the largest and most social species, tussled for control of the lookout. While the hyena was substantially larger than Homo erectus, the hominids had the advantage of tools such as projectiles. I suspect that more often than not, in an open site like Dmanisi, Homo erectus (a diurnal ape of tropical origin) would have come out on top of the nocturnal hyenas. In the dark of caves, however, the tables would almost certainly have been turned.
For about a million years after the Dmanisi individuals lived, evidence of Homo erectus is exceedingly rare in Europe. But we know from fossils preserved elsewhere that the species was becoming bigger-brained and was developing a more diverse toolkit. The next clear glimpse we get of Europe’s upright apes comes from the Sierra de Atapuerca in northern Spain, where caves have yielded fragmentary bones and tools dating to between 1.2 million and 800,000 years ago. The most important site, at Gran Dolina, provides strong evidence of cannibalism. The remains are mostly of juveniles, and they bear butchering and tooth marks.7 In 1997 these remains were named Homo antecessor. A few adult teeth and stone tools, dating to about 700,000 years ago, that were discovered in 2005 in cliffs at Pakefield, Suffolk, have been attributed to this species. Whether Homo antecessor is in fact just another form of Homo erectus remains an open question. I will be conservative and refer to it and all similar aged European remains as Homo erectus.
The discovery of Homo erectus remains in caves in Spain and Britain raises the issue of control of fire. Caves are cold and dark places that large carnivores prefer as lairs. The association of carnivores, but not most herbivores, with caves presumably relates to the amount of time an individual can stay under shelter. Carnivores kill infrequently, and spend days sleeping off their meal, while herbivores must spend most of their time foraging. Thus, except for hibernating species like cave bears, herbivores cannot benefit to the same extent from the more clement conditions provided by caves.