The Ancestor's Tale: A Pilgrimage to the Dawn of Evolution

by Richard Dawkins

The north-western part of Gondwana cracked from top to bottom, to one side South America, the other Africa, so that by 120 million years ago there existed an immensely long and narrow, angled channel between the two, which was to widen into the Atlantic Ocean of today.

Meanwhile, on the south-eastern part, India (with Madagascar attached) split off and started heading north. The final fragmentation kicked off 90 million years ago, when the remaining south-eastern land mass started splitting into Antarctica and Australia, while India divested itself of Madagascar, beginning a spectacularly fast mig...

Tectonic activity also had a profound effect on the climate, especially for the so...

Cretaceous Antarctica was mostly covered with temperate woodland, and a fine place for animals to live—not just warm-blooded mamm...

Volcanoes and ocean ridge eruptions meant carbon dioxide levels many times higher than today, resulting in a major ‘greenhouse effect’ of the ...

And Antarctica was particularly warm because of its Gondwanan connections. Land stretching from the pole to the equator directed warm ocean currents from the tropics to far southern latitudes, in a more dramatic version of the way...

From about 50 million years ago, as Antarctica’s land connections gradually disappeared, water began to circulate around the entire continent, increasing its thermal isolation...

There are two contrasting theories used to explain the distribution of related species. The first emphasises the importance of long-distance dispersal, such as the trans-oceanic colonisation of South America by monkeys and rodents. In contrast

‘vicariance biogeography’ emphasises the role of geological processes, such as shifting land masses, in separating biological populations.

an almost simultaneous split between xenarthrans, afrotheres and the northern hemisphere placental mammals—seems to fit rather neatly with the vicariance theory.

The last links between South America, Africa and Laurasia were severed at almost exactly the same time, when a land bridge from Brazil to West Africa, and another land bridge through Gibraltar, were inundated by a combination of land movement and rising sea levels.

The last time you could actually walk between the continents has now been pushed back to about 120 million years ago, substantially earlier than our estimate of 90 million years for Rendezvous 13.

Our conclusion is that it is the combination of dispersal and vicariance which is important. The unique South American fauna was probably a result of dispersal, yes, but dispersal gradually restricted by the slow separation of continents.

there were three extraordinary groups of mammals that radiated in South America following the demise of the dinosaurs, and, as it turns out, the other two also lend support to a mix of dispersal and vicariance.

South America. There they evolved various dog-, bear- and cat-like carnivorous forms. Unlike xenarthrans, they reached South America not from Africa but probably from North America, which is where earlier marsupial fossils are found.

The fossils pin this down to about 65 million years ago, when North and South America were not part of the same land mass (although there may have been a...

So the marsupials probably dispersed into South America over water too. The third group of these South American ‘old timers’—to borrow from the book Splendid Isolation by the great Ame...

These South American ‘notoungulates’ did the same herbivorous job as horses, rhinos and camels, but they evolved independently of them. The litopterns split early into horse-like and camel-like forms, which probably (from the position of the nose bones) had a trunk like an elephant. Another group, the pyrotheres, also seem to have had trunks, and may have been quite elephant-like in other respects.

The history of South America ended somewhat differently from Madagascar and Australia, those other gigantic rafts cut adrift by the breakup of Gondwana. Like India before it, South America’s island isolation came to an end naturally, before human travel brought all zoological isolation more or less to an end. It was a recent thing. The creation of the Isthmus of Panama, around 3 million years ago, led to the Great American Interchange.

No longer was dispersal a rare event. Instead, the separate faunas of North and South America were able to flow freely along the narrow corridor formed by the Isthmus, to each other’s continents. This enriched the two faunas, but then some extinctions occurred on both sides, presumably at least partly as a result of competition.

Since the Interchange there have been ground sloths in North America, as well as armadillos (including the enormous glyptodonts). Coming the other way, llamas, alpacas, guanacos and vicuñas, all members of the camel family, are now confined to South America, but camels originally evolved in North America. They spread into Asia and then Arabia and Africa quite recently, presumably via Alaska, where they gave rise to the Bactrian camels of the Mongolian steppe, and the dromedaries of the hot deserts. The horse family, too, did most of their evolving in North America but then went extinct there, which makes poignant the baffled surprise with which the Native Americans responded to the horses, reintroduced from Eurasia under the infamous conquistadores.

It is not just long-distance dispersal and the movement of continents which can explain species distributions. On a much faster timescale, the rise and fall of sea levels can also explain the links between the flora and fauna of adjacent regions.

the marsupials and the Australian flora and fauna that they represent. These species spread into Indonesia over land, at times of low sea levels during recent ice ages. Unlike the other examples in this tale, and for reasons unknown, their dispersal was stopped by narrow but deep ocean channels (perhaps there was not enough time to cross successfully, or the ecological niches on the other side were all occupied). Whatever the reason, a clear zoological and botanical line divides east from west Indonesia.

the Wallace Line. And the zone of transition—the islands between ...

Here we are, 160 million years ago, in the late Jurassic

the northern landmass of Laurasia had almost detached from the great southern continent of Gondwana, which itself was starting to tear down the middle, with Africa and south America to one side, and Antarctica, India and Australia to the other.

The climate was warmer than today, although snow and ice may have blanched the poles during the winter months. Only a few flowering plants grew in the temperate forests of coniferous trees and the plains of ferns that covered the northern and southern parts of the globe, and there were correspondingly few of the pollinating insects that we know today.

It is in such a world that the entire massed pilgrims of the...

all now represented by a small insectivore, greet the other great group of mammals, the marsupials.

Rendezvous 14 the divergence of the 340 or so species of marsupial from the 5,000 or so placental mammals.

It is not unlikely that all Australia’s marsupials stem from a single introduction of an opossum-like founder species from South America, via Antarctica. We don’t know exactly when, but it can’t have been much later than 55 million years ago, which is approximately when Australia (more especially Tasmania) pulled far enough away from Antarctica to be inaccessible to island-hopping mammals.

American opossums are no more closely related to the animals that Australians call possums than they are to any other Australian marsupials.

Most of the deep branches in the marsupial family tree, in other words, are American, which is one reason why we think the marsupials migrated to Australinea from America, rather than the other way around. But the Australinean branch of the family diversified mightily after their homeland became isolated. The isolation came to an end around 15 million years ago when Australinea (specifically New Gu...

Of approximately 340 surviving species of marsupial in the world, about three-quarters are Australinean (the rest are all American, mostly opossums plus a few other species such as the enigmatic Dromiciops, the monito del monte).

approximately 180 million years ago in the half-monsoonal, half-arid world of the Lower Jurassic.

The southern continent of Gondwana was still nestled closely against the great northern continent of Laurasia—the first time on our backwards journey that we find all major land masses collected into a more or less contiguous ‘Pangaea

The new pilgrims that join all the rest of the mammals here represent only three genera: Ornithorhynchus anatinus, the duckbilled platypus which lives in Eastern Australia and Tasmania; Tachyglossus aculeatus, the short-beaked echidna which lives all over Australia and New Guinea; and Zaglossus, the long-beaked echidna, which is confined to the highlands of New Guinea.

Collectively the three genera are known as monotremes.

According to one supported theory, long before the demise of the dinosaurs, the mammals were split into two major groups called the australosphenidans and the boreosphenidans.

Australo, once again, doesn’t mean Australian, it means southern. And boreo means northern, as in the northern aurora borealis. The australosphenidans were those early mammals that evolved in the great southern continent of Gondwana. And the boreosphenidans evolved in the northern continent of Laurasia, in a sort of earlier incarnation long before the evolution of the laurasiatheres we know today.

The monotremes are the only surviving representatives of the ...

Those therians who later became associated with the south, and with the breakup of Gondwana—for instance the afrotheres of Africa and the marsupials of South America and Australia—were boreosphenidans who had migrated south into Gondwana long after their northern origins.

even an animal that is genuinely primitive in all respects is primitive for a reason.

The ancestral characteristics are good for its way of life, so there is no reason to change. As my old tutor Professor Arthur Cain liked to say, an animal is the way it is because it needs to be.

The monotremes having joined us, the entire company of mammal pilgrims now walks back 140 million unbroken years, the longest gap yet between any two milestones, to Rendezvous 16 where we are to meet an even larger band of pilgrims than our own, the sauropsids: reptiles and birds.

Concestor 16, as we shall see, looked like a lizard (see plate 19). The gap from Concestor 15, which looked like a shrew, is too great to leave unbridged. We have to examine the mammal-like reptiles as shadow pilgrims, as though they were living pilgrims joining our march—although they shall not actually tell tales. But first, some background information on the timespan involved, because it is very long.

The intervening years without rendezvous milestones span half the Jurassic, the whole of the Triassic, the whole of the Permian and the final third of the Carboniferous. As the pilgrimage moves from the Jurassic back into the hotter and drier world of the Triassic—one of the hottest periods in the planet’s history, when all the land masses were joined together, forming Pangaea—we pass the late Triassic mass extinction, when three-quarters of all species went extinct.

But this is nothing compared to the next transition, from the Triassic Period...

At the Permo–Triassic boundary, a staggering 90 per cent of all species perished without descendants, including all the trilobites and ...

But the end-Permian mass extinction was the most devastating of all time. Some believe that, like the Cretaceous one, this was caused by a massive bolide collision, although the prevailing theory blames enormous episodes of volcanism. Even...