Universe a grand tour of modern science Phần 10

tree of life E A cited possible risks to human health and the environment. World Food Programme officials told them that what was good enough for 280 million Americans was good enough for them. To the question, ‘Who decides about transgenic crops?’ the answer seemed to be no one in particular. For more about natural gene mobility, see Tree of life. stagnant pool was a treat for Lynn Margulis when, as a young biologist at Boston University, she liked to descant on the little green bugs that can so quickly challenge human notions about how a nice pond should look. Her favourites included the blue-greens, often called algae but in fact bacteria, which have played an outstanding role in steering the course of life on the Earth. Margulis became the liveliest and most stubborn advocate of the idea that we are descended from bacteria-like creatures that clubbed together in the distant past. Others had toyed with this proposition, but she pushed it hard. In 1970 she published a book, Origin of Eukaryotic Cells, and she followed it in 1981 with Symbiosis in Cell Evolution. These are now seen as landmarks in 20th-century biology, and the keywords in their titles, eukaryotic and symbiosis, go to the core of the matter. You are the owner of eukaryotic cells. Each of the billions of microscopic units of which you’re built safeguards your genes of heredity within a nucleus, or karyon in Greek. So, in the grandest division of living things, into just two kinds, you belong to the eukarya. That groups you with other animals, with plants, with fungi, and with single-celled creatures called protoctists, represented by 250,000 species alive today and often ambiguous in nature. The other great bloc of living things, the prokarya, are all single-celled, and the genes just slop about within them. The earliest forms of life on the planet were all of that relatively simple kind, meaning bacteria and similar single-celled 681 tree of life organisms called archaea. They ruled the world alone for half its history, until the eukarya appeared. Symbiosis means living together. The proposition for which Margulis first marshalled all the available evidence is that small bacteria took up residence inside larger ones—inside archaea, one would say now—and so formed the ancestors of the eukarya. Instead of just digesting the intruders, the larger cells tolerated them as lodgers because they brought benefits. The outcome was the microscopic equivalent of mermaids or centaurs. ‘The human brain cells that conceived these creatures are themselves chimaeras,’ Margulis wrote with her son Dorion Sagan, ‘—no less fantastic mergers of several formerly independent kinds of prokaryotes that together co-evolved.’ Oval-shaped units inside your cells, called mitochondria, are power stations that use oxygen to generate chemical energy from nutrients. They look like bacteria, they carry sloppy genetic material of their own, and they reproduce like bacteria. The same is true of chloroplasts, small green entities found in the cells of the leaves of plants. They do the work of harvesting sunlight and using water and carbon dioxide to produce energy-rich molecules that sustain plant life and growth. I A recount of the kingdoms In the Margulis scenario, the ancestors of the mitochondria and chloroplasts were indeed bacteria that took up symbiotic residence inside other single-celled creatures. The mitochondrial forebears were bacteria that had learned to cope with oxygen. When that element first appeared unbound in the ancient sea it was deadly dangerous, like bleach poured into the bacterial–archaeal communities. So bacteria that were adapted to it could offer their hosts protection against oxygen and also the ability to exploit it in new ways of living. Blue-greens, formally called cyanobacteria, were the ancestors of the chloroplasts. In their separate, bacterial existence, they had hit upon the most powerful way of using sunlight to grow by. It involved splitting water and releasing oxygen, and so the blue-greens were probably responsible for the oxygen crisis. But this smart photosynthesis also conferred on the hosts the capacity to generate their own food supplies. Host cell plus mitochondria made the ancestors of fungi and of protoctists. The latter included some distinguished by their capacity for swimming about, which became the ancestors of the multicelled animals. Host cell plus mitochondria plus chloroplasts made single-celled algae, and among these were the forebears of the multicelled plants. 682 tree of life Aspects of the scenario are still debated. Especially uncertain is how all of these cells came to organize their cell nuclei, and how they perfected the eukaryotic kind of cell division used in multiplication, growth and sex. The origin of the capacity for movement in protozoa, and its possible survival in the swimming tails of sperm, is also controversial. The broad brushstrokes of the symbiosis story are nevertheless accepted now. Not just as a matter of taste, but by verification. The kinship of identifiable bacteria with mitochondria and chloroplasts is confirmed by similarities in their molecules. Fossil traces of early eukaryotes are very skimpy until 1200 million years ago, but the molecular clues suggest an origin around 2 billion years ago, at a time when free oxygen was becoming a major challenge to life. In 1859 Charles Darwin described a ‘great Tree of Life, which fills with its dead and broken branches the crust of the Earth, and covers the surface with its ever branching and beautiful ramifications’. He meant a family tree, such that all extinct and living species might be placed in their relative positions on its branches and twigs. As it was pictured in those days, the plant and animal kingdoms dominated the tree. The symbiosis theory redefines the main branches of the tree, with more kingdoms. Bacteria and archaea, sometimes lumped together as prokarya or monera, originate near the very base, when life began. Half-way up the tree, symbiosis introduces the peculiar and wonderful microbes called protoctists, which include the single-celled animal-like amoebas and plant-like algae. Other protoctists, with multifarious characters that are hard to classify, represent obvious experimentation with symbiosis. A boat-like microbe inhabiting the digestive tract of termites in Australia, and one of Margulis’ prime exhibits, has recruited some 300,000 wiggly bacteria to row in unison like galley slaves. Membership of the animal and plant kingdoms is, in this new tree, confined to multicelled creatures, so excluding amoebas and the other protoctists. The fungi, which include yeasts and moulds as well as mushrooms, get a kingdom of their own. They thus rank alongside the animals and plants, but are distinguished from them by their lack of embryos. Fungi are very important in decomposing dead plants and weathering the rocks. But in view of the diversity of protoctists, there is something odd about singling out the fungi for special status. What about the algae, which nowadays totally dominate life on most of the Earth’s surface—meaning the upper film of the wide oceans? I Bacterial sex and gene transfers As scientists trace the course of evolution more precisely than ever before, the more confused it becomes. To Darwin’s way of thinking, and for 100 years after 683 tree of life him, the branches and twigs of the evolutionary tree of life represented distinct lines of descent. If different branches traced back to common ancestors, those existed in the past, and after them the hereditary pathways were quite separate. That was supposedly guaranteed by the fact that any mating between different species was sterile. Organisms classified, grouped and named, according to their similarities and differences, hung on the Darwinian tree of life like Christmas presents. Each was in its proper place, with its label in Latin attached. But a bacterial guest in a symbiotic cell introduces into its host an inheritance from a completely different part of the tree. No longer do genes flow exclusively along a branch. They can also travel sideways from branch to branch, like tinsel. Some scientists call this lateral, others horizontal gene transfer. Was gene transfer a rare event? If it concerned only the invention of cells of the modern eukaryotic kind, 2 billion years ago, it might be seen as a rare historical quirk. But even before Margulis proclaimed evolutionary symbiosis, contemporary gene transfers between different lineages had turned up in hospitals. After antibiotics came into medicine in the 1940s, doctors were appalled by how quickly strains of harmful bacteria outwitted the miracle drugs. Pharmacologists are still in a non-stop race in which each new antibiotic soon meets resistant strains. Hospitals have become superbug factories where patients may die, if not by the infections themselves, then by toxic antibiotics given as a last resort. Human beings did not invent antibiotics. They are ancient poisons used in conflicts among microbes. In England during the Second World War, the pioneers of penicillin therapy simply harvested the material from cultures of a well-armed mould. From the point of view of the bacteria, it was not an unprecedented challenge, and some already possessed genes that conferred resistance. Evolve or perish—and evolve the bacteria did, at a startling rate, by distributing the genes for antibiotic resistance like insurance salesmen. Genes can pass from one bacterium to another, and even to different strains or species. In a primitive form of sexual behaviour, one bacterium simply injects genes into a neighbour. A virus invading one bacterium may pick up a gene there and carry it to another. Or a bacterium can simply graze on stray genes liberated from a dying cell. Nor are bacteria the only organisms open to gene transfers, by natural genetic engineering. In animals, a gene can be transcribed into an RNA virus, which does not even use the usual DNA in its genetic code, and then be translated back into DNA when the virus infects a new cell. Unhappily some genes transferred by this reverse transcription cause serious diseases. 684 tree of life While medical concerns multiplied with these discoveries, fundamental biology was in some disarray. A basic assumption had been that organisms resemble their parents. Any alterations in the genes occurred by mutation within an organism and were passed on by the normal processes of reproduction. Evolution supposedly accumulated changes in an ancestral lineage that was in principle, if not always in practice, clearly definable. The symbiotic origin of our cells, the genetically promiscuous bacteria, and reverse transcription too, showed these assumptions to be naıve. How important have gene transfers been, in evolutionary history? The answer to that question had to wait until molecular biologists worked out the tree of life for themselves, and examined complete sets of genes—the genomes—of present-day animals, plants and microbes. Then they could begin to trace individual genes back to their origins. I Doing without fossils The notion that one could discover the course of evolution from molecules germinated around 1960. That was when Brian Hartley at the Laboratory of Molecular Biology in Cambridge noted that poisons analogous to military nerve gases blocked the action of a wide variety of active proteins—enzymes—besides those involved in the control of muscles by nerves, which were the prime target of the nerve agents. He suspected that the various proteins had a common genetic ancestry. X-ray analyses showed how various proteins were shaped, and confirmed the idea. For example, three enzymes involved in human digestion, trypsin, chymotrypsin and elastase, turned out to have very similar structures. Other scientists compared proteins serving the same function, but in different species. Richard Dickerson of Caltech studied cytochrome C, which occurs in all plants, animals and fungi as an enzyme for dealing with oxygen. To perform correctly, it must have the same properly shaped active region, built by a particular sequence of subunits, amino acids, in the protein chain. But non-critical parts of the molecule could vary, and Dickerson found that by counting the differences between one species and another he could tell how closely they were related. For example, compared with cytochrome C in pigs, the same enzyme in chicken differs in 9 amino acids, in tuna in 17, and in cauliflower in 47. This was supermarket evolution. Instead of hammering on chilly rock faces, or wandering across searing deserts in search of fossils, you could collect your specimens in a basket at a local shop. If you felt more energetic you could catch a passing moth or frog to extend the scope of the investigation. You could then begin to construct a tree of life from the variable molecules in living organisms. 685 ... - tailieumienphi.vn