1.
THE COMMON THREAD
You are a third mushroom
âHow extremely stupid not to have thought of that.â
âTHOMAS HUXLEY,
ON HEARING OF DARWINâS THEORY OF
EVOLUTION BY NATURAL SELECTION
YOU ARE ONE THIRD mushroom. Thatâs right. You, me, all of us share a third of our DNA with fungi (as if my Christmas-card list was not long enough already!). This is strong evidence that humans and mushroomsâin fact all creatures that share the earth todayâhave a common ancestor. The person who first recognized this was the English naturalist Charles Darwin.
In 1831, aged just twenty-two, Darwin took up the post of shipâs naturalist on HMS Beagle. During its five-year voyage, he made a series of striking zoological observations. He noticed, for instance, that the birds and animals on the isolated GalĂĄpagos Islands, 1,000 kilometers off the west coast of South America, appeared to be variants of a small subset of birds and animals found on the continent. Not only that, but the birds and animals on each island of the GalĂĄpagos archipelago also differed from each other in subtle ways. Most famously, the finches that lived on islands where large nuts were available had stubbier beaks than finches on other islands.
After eighteen months of intense concentration, a light went on in Darwinâs mind. He realized why creatures were so exquisitely tailored for their environments. And it was not, as was the prevailing view, that they had been âdesignedâ by a Creator. There was a perfectly natural mechanism that created the âillusion of design.â
Most creatures, Darwin recognized, produced many more offspring than could be supported by the available food and were therefore destined to starve to death. However, in the struggle for survival, those individuals best suited to exploit the resources of their environment persisted, whereas those least suited perished. The casualties were staggeringly huge. But, by this process of evolution by natural selection, creatures changed incrementally, generation by generation, to be better adapted to their environments.
Darwin reasoned that, millions of years before, when the volcanic GalĂĄpagos Islands had risen from the sea, a handful of creaturesâbirds that had flown and other animals that had been driven by storms across the ocean on mats of vegetationâhad reached the archipelago from the mainland of South America. Finding an essentially empty world, they had spread out to fill all the available ecological niches. Darwinâs finches, isolated on different islands, had suffered the pressure of natural selection; the least adapted for survival had been brutally culled while the best adapted had prevailed. In the case of an island with large nuts, inevitably the finches that survived were variants with tough stubby beaks, perfect for cracking open big nuts.
Darwinâs courage was to present his theory of evolution by natural selection without knowing two key things: first, how characteristics were passed on, or inherited, from generation to generation; and, second, what created the variation in offspringâthe raw material for natural selection to work on. We now know that these two things are intimately connected. The blueprint for an organism is recorded in the large biological molecule called deoxyribonucleic acid, or DNA, which is carried in every cell.1,2 And it is mutations in DNA, often caused during the copying process, when cells reproduce, that give rise to varied and novel traits in offspring. âThe capacity to blunder slightly is the real marvel of DNA,â said the American biologist Lewis Thomas. âWithout this special attribute, we would still be anaerobic bacteria and there would be no music.â
According to Darwin, all creatures on Earth today have evolved by a process of natural selection from a simple common ancestral organism. This, ultimately, is the reason why we share one third of our DNA with mushrooms. In fact, the following stretch of DNA is present in every cell of every creature on Earth, including every one of the one hundred trillion cells in your body: GTGCCAGCAGCCGCGGTAATTCCAGCT CCAATAGCGTATATTAAAGTTGCTGCAGTTAAAAAG.3 Can there be a more striking piece of evidence that all creatures are related and that they evolved from a common ancestor, exactly as Darwin claimed? In the words of Thomas: âAll of todayâs DNA, strung through all the cells of the earth, is simply an extension and elaboration of the first molecule.â4
Darwin knew that the process of evolution by natural selection was painfully slow and would have required hundreds of millions, if not billions of years to create the profusion of life on Earth today. The first tentative evidence of life on our planet dates to about 3.8 billion years ago. Conceivably, the first cellâdubbed the âlast universal common ancestor,â or LUCAâarose around four billion years ago, a mere half a billion years after the birth of the earth. Exactly how this happenedâand how the step from nonlife to life was takenâremains one of the biggest unanswered questions in science.
2.
CATCH ME IF YOU CAN
Some slime molds have thirteen sexes
âI admit, I have a tremendous sex drive.
My boyfriend lives forty miles away.â
âPHYLLIS DILLER
SOME SLIME MOLDS HAVE thirteen sexes. (And you think you have trouble finding and keeping a partner!) Their sex cells, unlike human sperms and ova, which are hugely different in size, come in only one size. The gender of the cells is instead determined by three genes known as MatA, MatB, and MatC, which come in a number of variants. In fact, there are so many variants that potentially it is possible to have more than five hundred different sexes. To reproduce, a slime-mold spore must simply find a partner with different variants of its three genes.1
Nobody knows why some slime molds have thirteen sexes and some five hundred-plus. But then nobody knows why we have two sexes. Nor, for that matter, why we have sex.
In evolutionary terms the name of the game is to get your genes into the next generation.2 Not some of your genes but all of them. The sensible thing would therefore be to clone yourself since this ensures the transference of 100 percent of your genes to any offspring. Such asexual reproduction is in fact what most creatures on Earth practice. Organisms that have sex, on the other hand, pass on only 50 percent of their genes to the next generation. This means not only that they must give birth to twice as many offspring to achieve the same as asexual organisms but they must expend extra energy finding a partner as well. Sex appears to make no sense at all.
Many explanations for sex have been proposed but, until recently, none has been convincing. One, however, has now gained increasing acceptanceâand, surprisingly it concerns parasites.
Across the world at any one time, more than two billion people are unfortunately infected with parasites, which range from intestinal worms to malarial parasites. Such parasites tend to be small and able to reproduce quickly, which means they can go through many generations during the lifetime of their host. As a consequence, they can quickly adapt to their host so that they efficiently exploit its resources. The exploitation of those resources, however, is at the expense of their host, which is not only weakened but sometimes even killed.
Understanding what sex has got to do with parasites takes a bit of background. Imagine the DNA of an organism to be like a deck of cards. When the organism clones itself, its offspring inherit the entire deck of cards with maybe one or two cards slightly changed due to a random mutation. By contrast, in sexual reproduction, offspring inherit half a deck of cards from one parent shuffled together with half a deck of cards from the other parent. This makes the offspring not only different from either parent but also utterly unique. Consequently, the parentsâ parasites find themselves ill-adapted to the offspring and die.
The idea that sex continually wrong-foots parasites was proposed by the American biologist Leigh Van Valen in 1973.3 Basically the idea is that, although parasites can change rapidly, a host population can survive their relentless onslaught by changing even more rapidly.
In Through the Looking Glass, Lewis Carrollâs 1871 sequel to Alice in Wonderland, Alice runs beside the Red Queen but cannot understand why she is making no progress.
âIn our country,â said Alice, still panting a little, âyouâd generally get to somewhere elseâif you run very fast for a long time, as weâve been doing.â
âA slow sort of country!â said the Queen. âNow, here, you see, it takes all the running you can do, to keep in the same place.â
Van Valenâs parasite explanation for sex, which has become known as the âRed Queen Hypothesis,â received strong observational support in 2011.4 By genetic manipulation, biologists in the United States engineered two different populations of the roundworm Caenorhabditis elegans to reproduce in two different ways: one asexually, by fertilizing its own eggs; the other sexually, by female and male worms mating.5 The biologists then infected both groups of worms with a pathogenic bacteria. Serratia marcescens rapidly drove extinct the self-fertilizing population of worms but not the sexually reproducing ones. These outpaced their coevolving parasitesâthey continually ran faster. So, although it may not be the most romantic explanation for falling in love, defense against parasites seems to be the reason for sex.
3.
THE OXYGEN TRICK
Babies are powered by rocket fuel
âIn every one of us there is a living process of combustion going on very similar to that of a candle.â
âMICHAEL FARADAY1
A BABY SQUIRMS IN A cot. A rocket climbs high in the sky on a column of smoke and flame. Not much connection between them, you might think. But you would be wrong. Both are energized by the same chemical reaction. Both are powered by rocket fuel.
This is actually not as surprising as it may appear. Boosting a heavy rocket into orbit requires the most powerful fuelâthe one that, pound for pound, packs the biggest oomph. Life on Earth has been engaged in almost four billion years of trial-and-error experimentation. It would be odd if, in the attempt to power biological processes, it had not stumbled on the most potent energy source possible.
That energy source is the chemical reaction between hydrogen and oxygenâor combustion, as it is more commonly known. In the case of all animals, the hydrogen is extracted from food and the oxygen from the air. In the case of a rocket, liquid hydrogen and liquid oxygen are supplied by humans. Understanding how the reaction between hydrogen and oxygen works and where the tremendous energy comes from requires a little background.
Atoms of hydrogen and oxygenâin fact, all atomsâconsist of a tiny nucleus and even tinier electrons. The electrons orbit the nucleus, snared by its powerful electric force, ...