Adam, Eve and Uncle Albert

Patterns in Evolution - Darwin's Dreampond
六月 13, 1997

The cold war was good to physics. Its postwar scientific funding acquired the enviable status of having "national defence" implications and thereby dwarfed that of other disciplines: the expenditure on a single particle accelerator might fund an entire research department in another field. The price of physics is great enough even to have humbled America, whose Solons have backed away from building a giant accelerator ring somewhere in the Texas outback. This accelerator was to have been strong enough to recreate the physical equivalent of being within femtoseconds of the big bang - close enough to the beginning of time to lead some physicists to announce modestly that they were on the verge of understanding God's thoughts.

After decades of quietly languishing in shame for the mistakes of the eugenicists and fascists and after decades of getting by on hunting and gathering for research funds, it is genetics and not physics that is on the verge of literally spelling out the grand plans of life. This goes to show that governments are not very good at knowing what to pour money into, and what will matter years hence. DNA testing, the human genome project, genes for this or that disease or for this or that ability, and now cloning, are the scientific achievements of the 1990s that will most change the way we live, how we think about ourselves, and which will define the most knotty ethical issues. The prospect that one or two of Uncle Albert's cells can be salted away, one day to make a new copy of him, can no longer be treated blithely in after-dinner discussion. Indeed, an enterprising Seattle man will sell you your immortality for about Pounds 30. You get a matchbox-sized black headstone - uncomfortably reminiscent of Arthur Clarke's monolith in 2001: A Space Odyssey - in which are deposited a few of your DNA-containing cells awaiting Lazarus-like to be cloned in some future society.

Genetics' "big bang" occurred long after its birth but had the same effect of propelling the universe of genetic studies outwards at high speed. In December 1986, Kerry Mullis, along with several other authors, quietly published their results for a new and esoteric technique known as the "polymerase chain reaction". Whether the name for the technique deliberately borrowed terms from physics will have to await Mullis's autobiography. What is clear is that in the year following publication, Mullis's paper was cited three times, rising to 70 in 1988 and leaping to 256 in 1989. By 1991, the paper was being cited a startling 2,500 times a year and it is projected to be cited close to 7,000 times this year. The surf-boarding Mullis several years ago won the Nobel prize for what is now known simply as PCR and has achieved the status of a genetics guru, only partly aided by his, at times, erratic behaviour and scientific views.

What is PCR? The polymerase chain reaction for the first time allowed geneticists to amplify as little as a single molecule of genetic material (typically a length of DNA) into millions of copies. It then becomes possible to analyse the DNA, including the identification of the precise sequence of its genetic code, thereby allowing researchers to read the book of that particular stretch of DNA. The human genome project is an attempt to read - and understand - the whole "book of humans". The polymerase chain reaction is cheap and easy to perform - any genetics lab is capable of implementing it - and thus in a little more than ten years it has transformed genetics and its applications in biology. Irritatingly, "to PCR" has arrived as the newest infinitive in the scientific language.

Into this freshly transformed universe of genetics and its applications strides Roger Lewin's handsomely produced Scientific American book on patterns in molecular evolution. One should not be led by the book's title, Patterns in Evolution: The New Molecular View, to believe that this is a narrow scientific monograph, nor should one think that it will be hard going. Lewin is, and has for many years been, one of the more engaging and well informed of the scientific writer-journalists. In this book he is interested to explore how gene-sequence analyses are yielding discoveries in anthropology, evolution, and ecology. Accompanying Lewin's clear reportage is the usual Scientific American library dedication to producing books whose use of photographs and graphical displays is unmatched. Would that other publishers would take so seriously the presentation of their books.

Lewin swiftly develops the themes that Mullis's PCR has shifted from the possible but painstakingly tedious to the commonplace and now widely researched and influential. Potentially one of the molecular geneticists' most powerful tools is the "molecular clock". If genetic sequences evolve over long periods of time in some sort of clock-like or regular way, the observed genetic differences among pairs of species can be used to estimate when in the past they diverged. This is more than an academic exercise. The so-called Cambrian explosion describes a fantastical period of 10-20 million years in the fossil record beginning about 545 million years ago. During this time there is observed an explosive increase in the number of different animal forms. All living phyla today are seen in the Cambrian sediments. Weird and wonderful forms they are, too, with equally weird names such as Hallucigenia and Wiwaxia. So rapidly do new forms appear in the sedimentary layers during the Cambrian explosion that even Darwin reluctantly accepted that gradualistic (that is, Darwinian) step-by-step evolutionary change, in which natural selection aimlessly sorts among randomly varying individuals to find the most fit, could not provide an explanation. These new forms must have arisen in revolutionary and saltatory leaps in which wholly new forms appeared out of nowhere to fill some ecological niche.

The hegemony of the palaeontologists who have used the Cambrian explosion as a stick with which to beat the Darwinian gradualists is on the verge of being toppled by an application of the molecular clock that never even casts a glance at the fossil-rich Cambrian sediments, much less gets out a hammer to dig among them. Using segments of "ribosomal DNA" that evolve at the unimaginably slow rate of less than one change to a given site of a gene sequence every million years or so, molecular geneticists are finding that they can reliably distinguish at a genetic level the contemporary descendants of some of these most ancient forms known to biology. Armed with a molecular clock calibrated from known fossils of 100-300 million years of age, these new molecular palaeontologists are arriving at a startling conclusion: the differences among the contemporary forms are so large that the so-called Cambrian explosion must have happened long before the Cambrian. Worse for the "explosion" crowd, far from being an explosion it may have been closer to what one group of Oxford researchers is labelling a "slow-burn". Plenty of time for gradualistic evolution. Darwin must be grinning.

In some instances genetic information anchors a scientific claim arrived at by other means. "Out of Africa" is not just a Hollywood film, it is a contentious scientific hypothesis. Believers hold that the hominid lineage that would eventually lead to modern humans evolved once in Africa, later migrated out where it evolved into what we now recognise as Homo sapiens, and then spread around the world on the strength of its supreme intelligence. "Multi-regionalists", by contrast, believe that the premodern human left Africa, populated the world and then, independently, in many places on the globe, evolved into modern humans. This raises the socially delicate possibility that there may be only one real modern human and the rest were pretenders - among many independent realisations of some form, all are unlikely to be equal. Thankfully, palaeontological evidence largely favours the more palatable "out of Africa" view.

But what keeps palaeontologists awake at night is the fear that one's theory is only as good as the most recent fossil find: a single new bone can overturn the ruling dogma. Once again genetics can offer an opinion that is virtually independent of the palaeontological evidence. Researchers are gathering gene sequence information from people widely distributed around the world and applying techniques of analysis to reconstruct probable genealogies of modern humans. Genealogies derived from bits of "mitochondrial" DNA, a genetic sequence inherited only down the female line, place Eve somewhere in Africa around 250,000 years ago. Reassuringly, new studies of variation in the Y-chromosome also place Adam there at about the same time. Geneticists and palaeontologists largely agree on the question of human origins.

Lewin's book intersects Tijs Goldschmidt's book, Darwin's Dreampond, in the pond of Goldschmidt's title, Lake Victoria in East Africa. Goldschmidt tells a narrative tale spiked with table-top evolutionary theory of the extraordinary diversity of cichlid fish in this lake. Up until recently, Lake Victoria was home to at least 300 different species of cichlid fish. This is not the stuff of dreams per se - after all there are at least 500,000 different species of beetle - until it is realised that something of a mini-Cambrian explosion must have happened in this lake, perhaps within the past 200,000 years. Cichlids are normally small docile hand-sized fish that typically eat plants and plankton. The cichlids of Lake Victoria, however, have radiated into a bewildering variety of specialised forms that would be the envy of Dr Seuss, were he evil enough. There are fish-eating predatory cichlids, snail-crushing cichlids with specialised jaws, cichlids that, kamikaze-like, ram the throats of other cichlids to knock loose and consume the eggs that they brood in their mouths, snout-engulfing cichlids (horrifyingly, another way of dislodging and consuming eggs), and even a cichlid that specialises in plucking out and consuming the eyes of other cichlids. Life as a cichlid in Lake Victoria is not solitary but it is nasty, brutish, and often short.

How could so many different species of cichlid have evolved from a single founding species in such a short period of time, and later co-existed in what is a relatively small and homogeneous environment? Goldschmidt wonders if perhaps the various cichlids have not all evolved from a single ancestral stock, but rather represent a number of different radiations of the fish that have over the aeons found their way into the lake.

Traditional classifications of species based on their morphology pointed this way. Genetic investigations of the cichlids in Lake Victoria, however, suggest that they have all evolved from a recent common ancestor - so recent in fact that the different species of Lake Victoria are more closely related genetically than are populations of humans from around the globe. This then calls into question the whole enterprise: maybe these are not real species after all, but rather just variations of the same species.

So, the mystery of rapid speciation remains and Goldschmidt, frustratingly, has little to say about its resolution. The question probably never will be answered. The Nile Perch was introduced into Lake Malawi some years ago. This large and predatory fish has quite successfully "hoovered" up most of the species of cichlid in the Lake. As Goldschmidt laments, "one man with a bucket changed an entire ecosystem".

Lewin and Goldschmidt have produced very different books, but linked by the breathtaking advances in genetical techniques that make possible the investigation of questions that were out of reach only a few years ago. In other guises many of these techniques are the same as those that can land one in jail, settle a paternity dispute or help to trace one's ancestry. Recently DNA was retrieved from a well-preserved 9,000-year-old man found in a Somerset cave. For a lark, researchers thought: "why not test people in the area to see if any of this man's relatives are nearby". Nine thousand years on they found one of the man's descendants in the next village. Even this sort of thing may seem banal in the future if enough of those little immortality monoliths are sold.

Mark Pagel is senior research fellow in zoology, University of Oxford.

Patterns in Evolution: The New Molecular View

Author - Roger Lewin
ISBN - 0 7167 5069 4
Publisher - Scientific American Library
Price - £19.95
Pages - 246

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