These days, Mike Pole spends much of his time in Mongolia (where he often stays in a traditional, tent-like ger) and Kalimantan, the Indonesian part of the island of Borneo. He lives an adventurous life, dealing on a somewhat regular basis with floods, blizzards, and corruption; wading up streams in the Indonesian rainforest; and sleeping out under the stars in the Gobi Desert. Sarawak, where Alfred Russel Wallace wrote his paper “On the Law Which Has Regulated the Introduction of New Species,” the one in which he used geographic distributions of related species to argue for evolution, is also part of Borneo, but Pole isn’t there to follow in Wallace’s footsteps. Instead, as he says, he’s switched over to the “dark side” and now works as an exploration geologist, looking for coal. I was a little disappointed to hear this, not so much because of thoughts that coal is dirty energy, but because Pole wrote engaging, thought-provoking biogeographic papers; his 1994 paper on the New Zealand flora, in particular, influenced my own thinking about why things are found where they are. I thought he might have given up on that part of his career.
However, it turns out that Pole still lives part of the time in New Zealand, still finds the time to do research on fossil plants, and still thinks about the origins of the New Zealand flora. Like Gary Nelson, he believes that explaining the origins of New Zealand’s biota is a key to understanding the biogeographic history of the entire world. Of course, he also thinks that Nelson got the answer completely backwards; in Pole’s view, New Zealand points to the global importance of chance, overwater colonization, not ancient vicariance.
In his 1994 paper, Pole had made the strongest and most pointed case up to that time for the origins of New Zealand’s flora by oceanic dispersal. Nonetheless, a skeptic with a commitment to the vicariance worldview could have found holes in his argument. Pole, like Fleming and Mildenhall before him, had relied heavily on fossil occurrences, yet it is well known that the fossil record is incomplete. In particular, the apparent massive turnover in New Zealand’s flora since the birth of Zealandia could, in theory, be an artifact of a fragmentary record. Everyone agrees that New Zealand’s plant fossil record is far better than its vertebrate fossil record, but that doesn’t necessarily mean the plant record is reliable in an absolute sense. One possibility, for instance, is that many lineages persisted during certain periods only in small refugia where they were unlikely to leave any trace in the fossil record. Such groups could be Gondwanan relicts that Pole interpreted as later arrivals or as taxa that disappeared from and later recolonized New Zealand.
Similarly, Pole’s examples of “non-oceanic” Gondwanan plants that, against expectations, had colonized volcanic islands such as Norfolk and Lord Howe don’t indicate anything definite about the origins of New Zealand’s flora. The fact that oceanic dispersal can occur doesn’t mean that one should believe such a chance explanation when a more straightforward one—namely, persistence through Gondwanan breakup—would do just as well. If vicariance was the default explanation, as many biogeographers believed, then more direct evidence—something that would actually force one to reject the fragmentation hypothesis—was needed.
In that 1994 paper, Pole mentioned two sets of studies that were clearly outside of his own area of expertise, but supported his case for long-distance dispersal. These studies had to do with two iconic Gondwanan groups, the southern beeches and the ratite birds, and suggested relatively recent, overwater colonizations of New Zealand by at least some members of both groups. The authors of the ratite studies, for instance, claimed that the kiwi lineage had reached New Zealand from Australia within the past 45 million years, requiring a crossing of the Tasman Sea by these flightless birds.
In describing these two studies, Pole didn’t quite sound as if he fully endorsed them. His skepticism might have had something to do with the general approach the authors employed: they had assigned ages of colonization using a “molecular clock,” and the problems with that approach were well known. Problematic or not, however, those studies of southern beeches and ratite birds were heralds of what was to come. They were like the first drops of rain before the downpour.
19The name “Tasmantis” for this continent apparently has precedence over “Zealandia.” However, Zealandia is now the more widely used name.
20The list here is of the number of different taxa, not individuals, found in the Late Cretaceous fossil record of New Zealand (roughly 66 to 75 million years old). If the Paleocene, the first epoch of the Cenozoic, is added, the list includes two more kinds of birds, both penguins (Tennyson 2010).
21The assignment of Nothofagus fossil pollen to particular groups is questionable, and it is actually possible that none of the three groups mentioned existed at the time of the birth of Zealandia (Cook and Crisp 2005).
In July 1892, a natural floating island was spotted off the northeastern US coast, at about the latitude of Philadelphia and some 300 miles from the nearest land. The island was roughly 9,000 square feet in area, contained living trees 30 feet tall, and is said to have been visible from 7 miles away. The same island was again seen in September, by which time the Gulf Stream had pushed it more than 1,200 miles northeast of its previous position.
Section Two
TREES and TIME
Chapter Five
THE DNA EXPLOSION
A QUESTION OF TIMING
One of the stereotypically tedious parts of school history classes is committing to memory parts of the historical chronicle, that is, lists of events and their associated dates, all placed in chronological order. The Battle of Hastings in 1066, Columbus’s discovery of the New World in 1492, the signing of the Declaration of Independence on July 4, 1776—these are all familiar entries in the chronicle. Although memorizing such dates may be tiresome, knowledge of the chronicle is clearly vital to making sense of history, to understanding how and why things have happened. Just consider the sorts of historical connections we might contemplate if we had no idea about the proper chronological order of events. We might imagine that Magellan used Captain Cook’s maps to chart a route across the Pacific, or wonder if the financial meltdown of 2008 helped promote the rise of Hitler and the Nazi Party.
For human history, especially recent human history, such examples sound silly; the chronicle is typically so well established that we don’t waste time considering connections that violate the sequence. However, if history is considered in the most general sense—the history that includes cosmology, geology, evolutionary biology, linguistics, and other areas—there are many cases in which the absolute and relative timing of events is not an obvious collection of facts, but instead is something quite difficult to establish. For instance, starting with the first strong evidence for the Big Bang Theory, in the 1920s, estimates of the age of the universe have ballooned in a series of steps, increasing from about 2 billion years to 13.8 billion years.22 Similarly, although we can be sure that human language arose after our evolutionary separation from the chimp lineage, support for a more precise age for this critical event has been elusive. Obviously, there’s no written record of the Big Bang, or the origin of human language, and the evidence that does exist for dating these events can be hard to interpret. As a result, the possible chains of cause and effect are also often unclear. It has been suggested, for example, that the origin of language gave rise to selection for increased brain size, but that connection remains speculative, in part because of the uncertainty about exactly when language (or, more precisely, certain steps in the evolution of language) arose. Continuing our Hitler analogy, it’s as if we are often operating without clear knowledge of whether the 2008 financial crisis came before or after the Third Reich.23
Historical biogeography, like all historical disciplines, would benefit greatly from having an established chronicle of relevant events. More specifically, the problem of explaining piecemeal distributions fairly screams out for such timing information on the age of evolutionary branching points. In many of t
hese cases, the competing explanations involve processes taking place at different periods; typically, one is an ancient vicariance event, such as the opening of the Atlantic Ocean, and another involves the more recent dispersal of organisms across an ocean or some other barrier. So, if you knew when two lineages split—say, a group of rodents living in South America and another in Africa—there’s a good chance you would be able to either reject or support the ancient vicariance explanation; the split could be too recent to be explained by vicariance, or, alternatively, it could be old enough to fit that hypothesis. Biogeographers of all persuasions agree that having accurate ages for branching points in the tree of life would be enormously useful; all agree that knowing when would go a long way toward figuring out how. What they have conspicuously failed to agree upon is the practical role of such timing information in real studies of biogeography.
In the early 1990s, when Mike Pole was forming his ideas about the origins of the New Zealand flora, historical biogeography was deeply divided over this issue. It was like a nation polarized into two warring political parties, along with a large number of people of undecided allegiance. On one side were the hard-core vicariance scientists, including Gary Nelson and other cladists, along with panbiogeographers like Michael Heads, whose focus was on cladograms (without connected age information) or tracks as the fundamental kinds of evidence. They were notably disinterested in using fossils to place ages on evolutionary groups, a disinterest stemming from their belief that the fossil record is too incomplete to provide useful information for that purpose. In the extreme, these were the scientists who were entertaining ancient vicariance explanations even for such things as the origins of the Hawaiian biota or the distribution of Homo sapiens. They typically believed that the only good way to infer the age of an evolutionary branching point was to connect it to some fragmentation event, tectonic or otherwise. One could “know,” for instance, that Australian and New Zealand southern beeches had split from each other roughly 80 million years ago if, by looking at cladograms, it was established (and I use that term loosely) that their separation had been caused by Gondwanan breakup. In this view, time—the age of a branching point—was never used to discriminate between dispersal and vicariance, but instead was an outcome of already “knowing” that vicariance was the explanation.
On the other side were those who thought they had a rough handle on when many groups first appeared on Earth (and in particular places) and were willing to use that information to interpret biogeographic history. Think of those classroom posters that show the geologic time periods with the history of life superimposed on them—the first insects (crawling next to the word “Silurian”), the first mammals (Triassic), the first birds (Jurassic), and so on. This second group of scientists basically was made up of believers in such timelines, at least as approximations. Not surprisingly, most of them either were paleontologists or had a strong interest in the fossil record. They were people like Mike Pole and Dallas Mildenhall, immersed in the paleobotanical record of Zealandia; Anthony Hallam, a paleontologist who studied molluscs and other shelled invertebrates; and John Briggs, who did research on both living and fossil fishes. They weren’t ignorant about the effects of continental drift; in fact, both Hallam and Briggs wrote books about plate tectonics and its revolutionary impact on biogeography. However, all of them were arguing, as New York School dispersalists like William Diller Matthew, George Gaylord Simpson, and Ernst Mayr had decades before, that the fossil record tells us that some groups—a lot of groups, actually—are probably too young to have been split up by ancient fragmentation events. For example, Briggs, in his 1987 book Biogeography and Plate Tectonics, suggested that a group of freshwater killifishes found in Africa and South America might have come into existence long after the opening of the Atlantic. If that was the case, at least one of these fishes must have crossed the ocean, somehow tolerating the salty waters. Like others in this school of thought, Briggs had no particular preference for vicariance or dispersal as explanations; he would follow the evidence, and for him, the evidence included information about the ages of groups.
At the same time, many biologists who had or might have had some interest in biogeography were not in either camp. Some of these people weren’t ready to accept the extreme views of the vicariance side, yet were also leery of putting too much faith in the fossil record. Michael Donoghue, the botanist who had been attracted to Gary Nelson’s intellectual boldness, was one of those. Despite being “raised” as a cladist, he was turned off by the endless, inconclusive cladograms in Nelson and Platnick’s vicariance tome, Systematics and Biogeography—“chicken scratchings,” Donoghue called them—yet he wasn’t ready to believe, à la Pole and Mildenhall, that long-distance dispersal was commonplace. Up to the early 1990s, Donoghue had mostly set aside an early interest in biogeography and was pursuing other things. Others had never really been drawn in to begin with, perhaps because the field, after the burst of enthusiasm following the revelation of continental drift, seemed a bit stagnant. However, I suspect that most of these “undecideds,” when they were thinking about biogeography at all, had leanings toward vicariance rather than dispersal, because vicariance seemed like the more global and cutting-edge idea. As I mentioned in the Introduction, I knew little about biogeography through the 1990s, but when I had to lecture about the subject in an evolution course, I chose to talk mostly about Gondwanan breakup, not ocean crossings. Global fragmentation just seemed like the “cooler” thing to focus on. In short, the undecided vote seemed, if anything, poised to tip toward the vicariance side.
What shifted this balance, particularly for the undecideds, was the use of molecular data, especially DNA sequences, to put ages on evolutionary branching points. Molecular dating had actually begun long before this, in the early 1960s,24 but such studies became much more widespread in the 1990s, and that upward trend has continued to the present. For many people, this approach suddenly made establishing the evolutionary chronicle a reality. It was like finally being able to show, after years of ignorance, that Hitler really had come to power decades before the financial crisis of 2008.
Given the importance of molecular dating for biogeography, a huge question that we have to deal with is whether the ages estimated in this way can really be trusted. Some evolutionary biologists, including, not surprisingly, some of the hard-core vicariance crowd, continue to think that molecular dating is basically worthless, and therefore, that any conclusions that depend on it are equally worthless. In a 2005 paper, Michael Heads wrote that “degree of divergence is a guide neither to the time involved in evolution, nor the age of that evolutionary [splitting] event,” and that the molecular clock approach “does not solve biogeographical problems but simply leads into a morass of mysteries and paradoxes.” Similarly, Gary Nelson, now retired but still quick with a witty phrase, has ridiculed the approach and its use in biogeography as a futile “molecular dating game.”
In Chapter Six, I will get into the thorny but critical issue of whether we should trust molecular dating studies. First, however, I want to address, somewhat idiosyncratically, the question of why this approach took off when it did. In a sense, there can never be a complete answer to a question like that; one can always delve deeper into the long sequence of historical cause and effect, or flesh out in greater detail what happened at key points. In the case of the molecular dating explosion, one could argue for the importance of things like the discovery of the structure of DNA, and, later, of the enzymes that make strands of DNA replicate themselves. The idea of the molecular clock itself, first proposed by an Austrian biochemist named Emile Zuckerkandl and the Nobel Prize–winning chemist Linus Pauling in 1962, and the invention in the 1970s of methods for obtaining long sequences of DNA, were also critical. However, I take all that as background and instead focus on a critical insight that a particular scientist had in the early 1980s. In doing so, I’m not subscribing to a “great man/woman” view of history. Rather, I’m simply emphasizing
an event that clearly had a rapid and far-reaching effect. This event may also qualify as a potential “point of no return,” that is, an occurrence that set off an unavoidable cascade of effects. Maybe I’m also biased by the fact that I experienced part of the effect of the event in question firsthand, within a few years of when it happened.
Such scientific turning points are not always memorably discrete, even to the people making them. For instance, although there may have been a particular moment when Darwin became a confirmed believer in evolution, his thoughts on the subject had been percolating for years; when he finally converted, it was like fitting a few pieces into a puzzle whose basic form he had already seen. However, if we can take the word of its architect, the turning point I’m about to describe did indeed come in a lightning-like epiphany. That epiphany, that flash point, can be located very precisely in time and space, to a night in May 1983, on Highway 128 in the Coast Range north of San Francisco, at mile marker 46.58.
THE CHAIN REACTION BEGINS
It was an unseasonably warm night, and the air was thick with the sweet scent of blooming California buckeyes. Kary Mullis was at the wheel of his silver Honda Civic, driving north from Berkeley toward his cabin in Anderson Valley, his girlfriend asleep in the passenger seat. He was thinking about DNA replication.
Mullis and his girlfriend were both chemists working for Cetus, a Bay Area biotech company that, among other things, developed cancer therapies and ways to diagnose genetic diseases such as sickle-cell anemia. Mullis was hardly your stereotyped dull corporate scientist in a white lab coat, however. In fact, he was (and is) a risk-taker and close to a certifiable nut. He had experimented with LSD and other hallucinogenic drugs, even brewing up new compounds and trying them out on himself. (A Berkeley professor in whose laboratory Mullis worked while getting his doctorate had once suggested, with surprising restraint, that Mullis might want to clear the psychoactive substances out of the lab freezer, in case the cops came around.) In Aspen, Colorado, he once skiied down the middle of an icy road with cars whizzing by on both sides, apparently unconcerned because he had it in his head that he would eventually be killed by crashing into a redwood tree, and there weren’t any redwoods in Aspen. Weirder still, he had once passed out while getting high on laughing gas, with the tube from the tank stuck in his mouth, and claimed that he was saved by a stranger, a woman who noticed his prostrate form as she floated by on the astral plane. Somehow, bodiless, she managed to get the freezing tube out of Mullis’s mouth. Parts of his lips and tongue were frostbitten, but he survived the incident and, years later, met the corporeal version of his savior in a bakery, as if by destiny.
The Monkey's Voyage Page 14
