Why Evolution Is True (32 page)

Polyploidy is much rarer in animals, appearing only occasionally in fish, insects, worms, and reptiles. Most of these forms reproduce asexually, but there is one sexually reproducing polyploid mammal, the curious red viscacha rat of Argentina. Its 112 chromosomes are the most seen in any mammal. We don’t understand why animal polyploids are so rare. It may have something to do with polyploidy disrupting the mechanism of X/Y sex determination, or with the inability of animals to self-fertilize. In contrast, many plants do have the ability to self-fertilize, which allows a single new polyploid individual to produce many related individuals that are all members of its new species.

Polyploid speciation differs from other types of speciation because it involves changes in chromosome number rather than changes in the genes themselves. It is also immensely faster than “normal” geographic speciation, for a new polyploid species can arise in just two generations. That is nearly instantaneous in geologic time. And it gives us the unprecedented chance to see a new species appear in “real time,” satisfying the demand to view speciation in action. We know of at least five new plant species that arose this way.

One is the Welsh groundsel
(Senecio cambrensis),
a flowering plant in the daisy family. It was first observed in North Wales in 1958. Recent studies have shown that it is in fact a polyploid hybrid between two other species, one of them the common groundsel
(Senecio vulgaris),
native to the United Kingdom, and the other the Oxford ragwort
(Senecio squalidus),
introduced to the UK in 1792. The ragwort didn’t appear in Wales until about 1910. This means that, given the British penchant for botanizing-which produces a continuous inventory of local plants-the hybrid Welsh groundsel must have arisen between 1910 and 1958. The evidence that it is indeed a hybrid, and arose via polyploidy, comes from several fronts. For a start, it looks like a hybrid, since it has features of both the common groundsel and the Oxford ragwort. Moreover, it has exactly the chromosome number (sixty) predicted for a polyploid hybrid with those two parents. (One parent has forty chromosomes, the other twenty.) Genetic studies have shown that the genes and chromosomes of the hybrid are combinations of those seen in the parental species. The final proof came from Jacqueline Weir and Ruth Ingram of St. Andrews University in Scotland, who completely synthesized the hybrid species in the laboratory by making various crosses between its two parental species. The artificially produced hybrid looks precisely like the Welsh groundsel seen in the wild. (Wild hybrid species are often resynthesized in this way to check their ancestry.) There is little doubt, then, that the Welsh groundsel represents a new species that arose in the last hundred years.

The other four cases of real-time speciation are similar. All involve hybrids between a native species and an introduced one. Although this involves some artificiality, in the form of humans moving plants around, it’s almost necessary to have this happen if we want to see new species form before our eyes. It seems that polyploid speciation occurs very quickly when the appropriate parental species live in the same place. To see an allopolyploid species arising in nature, then, we must be on the scene soon after its two ancestral species come into close proximity. And this will happen only after a recent biological invasion.

But polyploid speciation has occurred, unwitnessed, many times during the course of evolution. We know this because scientists have synthesized polyploid hybrids in the greenhouse that are virtually identical to those that formed in nature long before we were around. And the artificially produced polyploids are interfertile with the ones in the wild. All this is good evidence that we’ve reconstructed the origin of a naturally formed species.

These cases of polyploid speciation should satisfy those critics who won’t accept evolution unless it happens before their eyes.
⁴²
But even without polyploidy, we still have plenty of evidence for speciation. We see lineages splitting in the fossil record. We see closely related species separated by geographic barriers. And we see new species beginning to arise as populations evolve incipient reproductive barriers-barriers that are the foundation of speciation. No doubt Mr. Darwin, were he to awaken today, would be delighted to find that the origin of species is no longer a “mystery of mysteries.”

Chapter 8

What About Us?

-William S. Gilbert and Arthur Sullivan, Princess Ida

 

 

 

I
n 1924, while dressing for a wedding, Raymond Dart was literally handed what would become the greatest fossil find of the twentieth century. Dart was not only a young professor of anatomy at the University of Witwatersrand in South Africa, but also an amateur anthropologist, and had spread the word that he was looking for “interesting finds” to fill a new anatomy museum. As Dart was donning his tuxedo, the post-man brought him two boxes of rocks containing bone fragments excavated from a limestone quarry near Taungs, in the Transvaal region. In his memoir,
Adventures with the Missing Link,
Dart describes the moment:

The groom’s concern is understandable. Nobodywants to discover on their wedding day that their best man is more interested in a box of dusty rocks than in the impending nuptials. Yet it’s difficult not to sympathize with Dart as well. In
The
Descent of Man, Darwin had conjectured that our species had originated in Africa because our closest relatives, gorillas and chimpanzees, are both found there. But this was little more than a hunch. There were no fossils to back it up. And there was manifestly something of an evolutionary gulf between us and the common ancestor we must have shared with other great apes—an ancestor that was surely more apelike than human. On that day in 1924, the first stepping stone was uncovered, showing that the gulf would eventually be crossed: there it was, in Dart’s trembling hands, a direct glimpse of what had long before been simplistically dubbed the “missing link.” One wonders how he could have concentrated on his duties at the wedding.

What Dart found in that box was the first specimen of what he later named
Australopithecus africanus
(“Southern ape-man”). In the next three months, Dart’s meticulous dissection of the rock, using sharpened knitting needles purloined from his wife, revealed the full face. It was the face of an infant, now known as the “Taungs child,” complete with milk teeth and erupting molars. Its mixture of human and apelike traits clearly confirmed Dart’s idea that he had indeed stumbled upon the dawn of human ancestry.

Since Dart’s time, paleoanthropologists, geneticists, and molecular biologists have used fossils and DNA sequences to establish our place in the tree of evolution. We are apes descended from other apes, and our closest cousin is the chimpanzee, whose ancestors diverged from our own several million years ago in Africa. These are indisputable facts. And rather than diminishing our humanity, they should produce satisfaction and wonder, for they connect us to all organisms, the living and the dead.

But not everyone sees it that way. Among those reluctant to accept Darwinism, human evolution forms the core of their resistance. It doesn’t seem so hard to accept that mammals evolved from reptiles, or land animals from fish. We just can’t bring ourselves to acknowledge that, just like every other species, we too evolved from an ancestor that was very different. We’ve always perceived ourselves as somehow standing apart from the rest of nature. Encouraged by the religious belief that humans were the special object of creation, as well as by a natural solipsism that accompanies a self-conscious brain, we resist the evolutionary lesson that, like other animals, we are contingent products of the blind and mindless process of natural selection. And because of the hegemony of fundamentalist religion in the United States, this country has been among the most resistant to the fact of human evolution.

In the famous “Monkey Trial” of 1925, high school teacher John Scopes went on trial in Dayton, Tennessee—and was convicted—for violating Tennessee’s Butler Act. Tellingly, this law didn’t proscribe the teaching of evolution in general, but only the idea that
humans
had evolved:

While more liberal creationists admit that some species could have evolved from others,
all
creationists draw the line at humans. The gap between us and other primates, they say, was unbridgeable by evolution, and must therefore have involved an act of special creation.

The idea that humans are part of nature has been anathema over most of the history of biology. In 1735, the Swedish botanist Carl Linnaeus, who established biological classification, lumped humans, whom he named
Homo sapiens
(“man the wise”), with monkeys and apes based on anatomical similarity. Linnaeus didn’t suggest an evolutionary relationship between these species—his intention was explicitly to reveal the order behind God’s creation—but his decision was still controversial, and he incurred the wrath of his archbishop.

A century later, Darwin knew full well the ire he would face by suggesting, as he firmly believed, that humans had evolved from other species. In
The Origin
he pussyfooted around the issue, sneaking in one oblique sentence at the end of the book: “Light will be thrown on the origin of man and his history.” Darwin didn’t come to grips with the issue until more than a decade later in
The Descent of Man
(1871). Emboldened by his growing insight and conviction, and by the confidence gained from the rapid acceptance of his ideas, he finally made his views explicit. Mustering evidence from anatomy and behavior, Darwin asserted not only that humans had evolved from apelike creatures, but did so in Africa:

Imagine the effect of that sentence on Victorian ears. To think that our ancestors lived in trees! And
were furnished
with tails and pointed ears! In his last chapter, Darwin finally dealt head-on with the religious objections:

Nevertheless, he didn’t convince all of his colleagues. Alfred Russel Wallace and Charles Lyell—Darwin’s competitor and mentor, respectively—both signed on to the idea of evolution but remained unconvinced that natural selection could explain the higher mental faculties of humans. It took fossils to finally convince the skeptics that humans had indeed evolved.

IN 1871, the human fossil record comprised only a few bones of the late-appearing Neanderthals—too humanlike to count as a missing link between ourselves and apes. They were regarded instead as an aberrant population of
Homo sapiens.
In 1891, the Dutch physician Eugene Dubois turned up a skull-cap, some teeth, and a thighbone in Java that filled the bill: the skull was somewhat more robust than that of modern humans, and the brain size smaller. But distressed by the religious and scientific opposition to his ideas, Dubois reburied the bones of
Pithecanthropus erectus
(now called
Homo erectus)
beneath his house, hiding them from scientific scrutiny for three decades.