Evolution: Class 12 Biology · Chapter 6

Split a rock in the Narmada valley near Dhar and you might hold a fossil dinosaur egg about 15 cm across, laid near the very end of the age of dinosaurs. Then look at the pigeon on your balcony: it belongs to the one branch of the dinosaurs that is still alive. How do eggs in ancient rock connect to birds in today's sky? This chapter follows that thread and many others.

Evolutionary biology asks how the living world came to be as it is: where the first living things came from, why species change, and how the millions of species alive today are related. The answers span almost four billion years, so we begin even earlier, with the universe and the Earth.

6.1 Origin of Life

Light needs time to cross space, so a telescope is also a time machine: light reaching us from a galaxy billions of light-years away shows that galaxy as it was billions of years ago. Collecting the oldest light in the sky has let astronomers reconstruct the history of the universe and measure its age, about 13.8 billion years.

According to the Big Bang theory, the universe began in an extremely hot, dense state and has been expanding and cooling ever since. It was not an explosion into empty space: space itself stretched. Nuclei of hydrogen and helium formed within the first few minutes. Over the next few hundred million years gravity gathered the gas into stars and galaxies. Heavier elements, such as the carbon, oxygen and iron in your body, were forged inside stars and scattered when those stars died. Our Sun and its planets condensed from such a cloud in the Milky Way galaxy, and the Earth formed about 4.54 billion years ago.

The young Earth was hot, battered by meteorites and covered with volcanoes. Gases escaping from its interior, mostly water vapour, carbon dioxide and nitrogen with some hydrogen, methane and ammonia, formed an atmosphere with almost no free oxygen. With no oxygen there was no ozone layer, so strong ultraviolet light reached the surface. As the crust cooled, the water vapour condensed and fell as rain for an immensely long time, filling the hollows to make the first oceans.

Where did the first life come from? Two old answers avoid the hard part. Panspermia, an idea going back to Greek thinkers and revived by Svante Arrhenius in 1903, proposes that 'seeds' of life drifted to Earth through space. Even if true, it only moves the question to another planet. The second answer, spontaneous generation, held that living things arise from non-living matter all the time: maggots from rotting meat, frogs from mud, microbes from stale broth.

In 1668 Francesco Redi covered some jars of meat with gauze and left others open; maggots appeared only where flies could reach the meat. Microbes were harder to rule out, because they are invisible and everywhere. Louis Pasteur settled the question around 1860 with flasks whose necks he drew out into an S-shaped curve (Fig. 6.1).

The curved neck answered a critic's objection that boiling destroys some 'vital force' in the air: here fresh air reached the broth freely, yet nothing grew until microbes got in. The conclusion, that living things arise only from other living things (biogenesis), ended spontaneous generation as a scientific idea. It did not explain how the very first life began.

In the 1920s the Russian biochemist Alexander Oparin and the British biologist J. B. S. Haldane, working separately, suggested that life did arise from non-living matter, but only once, slowly, and under conditions unlike today's. The first stage was chemical evolution (रासायनिक विकास): lightning, ultraviolet light and volcanic heat drove reactions among simple molecules, building sugars, amino acids and nucleotide bases that collected in the seas as a dilute 'soup'. The lack of oxygen was essential, because oxygen would have broken these fragile molecules down.

In 1953 Stanley Miller, a student in Harold Urey's laboratory in Chicago, tested the idea (Fig. 6.2). He sealed methane, ammonia and hydrogen with water in a sterile glass loop. Water boiling in one flask sent vapour round into a large chamber where electric sparks stood in for lightning. A condenser cooled the gases, and the liquid that formed ran into a trap and back to the flask. Within a week the water had turned brown, and tests showed amino acids, among them glycine and alanine, made from nothing but simple gases and energy.

Variations of the experiment, with other gas mixtures and with ultraviolet light or heat in place of sparks, have since yielded the bases found in DNA and RNA, simple sugars and the fatty molecules that can form membranes. When Miller's stored samples were re-analysed with modern instruments in 2008 they held more than 20 different amino acids. Geologists now think the early atmosphere was less rich in methane and ammonia than Miller assumed, so researchers also study hot springs and deep-sea vents, where heat and minerals could drive similar chemistry. The central lesson holds: the molecules of life form readily from simple ingredients and a source of energy.

How did chemistry become biology? Here the evidence is thinner, and scientists reason from clues. A living cell needs a molecule that stores information and also speeds up reactions. RNA can do both: some RNA molecules (ribozymes) act as catalysts, and an RNA strand can serve as a template for copying. This points to an early 'RNA world' of self-copying molecules. Fatty molecules form hollow bubbles in water on their own, and RNA trapped inside such bubbles would make simple protocells. Once a protocell could grow, divide and pass on its RNA, natural selection could begin.

The oldest evidence of cells is about 3.5 to 3.7 billion years old: layered mounds built by microbial mats (stromatolites) and carbon with a biological signature. These were simple prokaryotes in the sea; the land stayed lifeless for billions of years more. That life emerged from non-living chemistry by natural processes is the working consensus of science (it is sometimes called abiogenesis); the exact route from protocell to true cell is still being researched.

6.2 Evolution of Life Forms – A Theory

An older view, special creation, held that species are fixed. On this view each kind of organism was made separately in its present form, no kind has appeared or vanished since, and the Earth is only some thousands of years old. Nineteenth-century geology and biology found evidence against each point: rock layers that took millions of years to build up, fossils of species that no longer exist, and living species that differ only slightly from one another.

Charles Darwin spent five years, 1831 to 1836, aboard HMS Beagle as it surveyed the coasts of South America and then sailed on round the world. In Argentina he dug up bones of giant extinct mammals that looked like outsized versions of the armadillos and sloths still living there. On the Galápagos Islands the birds and tortoises differed slightly from island to island, yet all resembled species of the South American mainland. Back in England he spent two decades testing one explanation: species are not fixed but descend, with gradual modification, from earlier species.

The process he proposed is natural selection. Picture deer mice living on pale sand. Their coat colours vary, the variation is inherited, and owls catch the mice that stand out. The well-matched mice raise more young, so the next generation is, on average, a little paler; repeat this for many generations and the population changes. Success here is counted in offspring. Darwinian fitness is reproductive success, how many surviving young an individual leaves, and not strength or speed for their own sake.

The theory has two big implications, both since confirmed. Every pair of species, a tiger and a mango tree included, shares an ancestor if you go back far enough, so all life forms one family tree (Fig. 6.3). And because such change is slow, the Earth must be ancient: radiometric dating now puts it at 4.54 billion years, not the few thousand once assumed. That is why the rock record doubles as a calendar of life.

6.3 What are the Evidences for Evolution?

Evidence for evolution comes from many independent sources: rocks, embryos, anatomy, molecules, breeders' records and change watched as it happens. Any one of them could have contradicted the idea. Instead, they tell the same story.

Fossils

A fossil (जीवाश्म) is the preserved remains or trace of an organism from the distant past. Most are hard parts, such as bones, shells, teeth or wood, turned to stone, but footprints, burrows and leaf impressions count too. Most fossils form in sedimentary rock, where mud and sand settle in layers that slowly harden. In undisturbed rock, each layer is younger than the one beneath it (Fig. 6.4).

Palaeontological evidence, the evidence from fossils, rests on the order of life in the rocks. The deepest fossil-bearing rocks contain only single-celled life. Higher layers add soft-bodied animals, then shelled invertebrates, fish, land plants, amphibians, reptiles, mammals and, in the very youngest rocks, humans. Each group first appears at a particular level, and many vanish higher up because they went extinct: dinosaurs other than birds are found only in rocks between about 230 and 66 million years old. A world of fixed, unchanging species would leave no such sequence.

Fossils also capture in-between forms. Tiktaalik, a fish with a neck and sturdy limb-like fins, comes from rocks about 375 million years old. Archaeopteryx, about 150 million years old, had feathers and wings but also teeth, clawed fingers and a long bony tail, linking small dinosaurs and birds.

How do we know these ages? By radiometric dating, which you met in physics. A radioactive isotope decays at a fixed rate set by its half-life, so the fraction left in a sample works as a clock. Carbon-14, with a half-life of 5730 years, dates remains up to about 50,000 years old. Older rocks need slower clocks, such as potassium-40 (half-life 1.25 billion years), used on the volcanic ash layers above and below a fossil.

Embryos

In the 1860s Ernst Haeckel claimed that an embryo replays the adult forms of its ancestors, so a human embryo would pass through a 'fish stage' and a 'reptile stage'. Careful comparison showed this is wrong. Decades earlier Karl Ernst von Baer had observed that the embryo of one animal resembles the embryos of related animals, never their adults, and that features shared by a large group appear earlier in development than features special to a smaller group.

What embryos do show is shared ancestry. Early in development every vertebrate embryo, whether fish, bird or human, has a series of pharyngeal arches separated by grooves in the neck region, often loosely called gill slits. In fish they develop into gill supports. In mammals the same embryonic tissues become parts of the jaw, the three tiny bones of the middle ear, and glands such as the thymus. One shared starting kit, reshaped differently in each group, is what descent from a common ancestor predicts.

Comparative anatomy: homology and analogy

Why should a bat's wing contain a thumb and finger bones? Nothing about flying demands them. In Fig. 6.5 the forelimbs of a human, a bat, a dolphin and a horse line up bone for bone, from a single humerus at the shoulder, through the radius and ulna, carpals and metacarpals, to the phalanges at the tip, although they grasp, fly, swim and run. The simplest explanation is inheritance: all four mammals received this limb plan from a shared four-legged ancestor, and selection then stretched, shortened or fused its parts. Such structures are homologous organs (समजात अंग), and the process that drives related forms apart is divergent evolution. The same reasoning applies inside the body: the brains of a frog, a crocodile and a human are built from the same three main regions.

Plants show homology too. The thorns of Bougainvillea and the tendrils of Cucurbita (the gourd family) look nothing alike, yet both grow from the axil of a leaf, so both are modified shoots. In the same way, the climbing tendrils of a garden pea and the spines of Opuntia are both modified leaves or parts of leaves.

Now compare a shark with a dolphin. Both are streamlined, with a fin on the back and a powerful tail, yet one is a fish and the other an air-breathing mammal whose skeleton and fins are built quite differently. Features that look alike or do the same job but differ in structure and origin are analogous organs (समरूप अंग). They arise by convergent evolution: when unrelated lineages face the same physical problem, such as moving fast through water, selection tends to arrive at similar solutions.

Molecules

The deepest similarities are chemical. All cellular life stores its genes in DNA, built from the same four bases, and reads them with almost the same genetic code into proteins made of the same 20 amino acids. Proteins with the same job, such as cytochrome c in respiration, have very similar sequences across species, and the differences grow with evolutionary distance. Comparing DNA gives the same family tree that anatomy and fossils suggest (Table 6.1).

Artificial selection

Breeders run a fast-forward version of the process. By choosing which plants or animals get to reproduce, people have reshaped crops and livestock within a few thousand years. One wild coastal plant of Europe, wild cabbage (Brassica oleracea), has been turned into six familiar vegetables simply by favouring a different part each time (Table 6.2). Darwin, a keen pigeon breeder himself, opened On the Origin of Species with exactly this kind of evidence: breeding reveals how much heritable variation populations hold, and nature has had vastly longer to work with it.

Natural selection observed

The peppered moth (Biston betularia) of England rests on tree trunks by day. Most individuals were pale and speckled, matching the pale lichens on the bark. In 1848 a dark (melanic) moth was recorded near Manchester. As coal smoke killed the lichens and blackened the trunks, pale moths stood out and were eaten by birds, while dark moths were hidden. By the 1890s about 98 % of the moths near Manchester were dark (Fig. 6.6).

This shift, called industrial melanism, ran in reverse after clean-air laws were passed in the 1950s: the lichens returned, the bark lightened and dark moths became rare again. In unpolluted countryside, dark moths had stayed uncommon all along. Field studies confirmed that birds take more of the moths that contrast with their background, and in 2016 geneticists traced the dark form to a single mutation in a gene called cortex that arose around 1819. Throughout, both forms persisted; what changed was their ratio. Lichens themselves are useful pollution indicators, because most species cannot grow in air polluted with sulfur dioxide.

People now drive evolution in many species, often by accident. Spray a field with the same pesticide year after year and the rare insects able to survive it leave nearly all the offspring; within a few seasons the pesticide fails. In India the pink bollworm evolved resistance to Bt cotton within about a decade of the crop's introduction. Weeds resist herbicides, mosquitoes resist DDT and bacteria resist antibiotics in the same way (the steps below). Biologists call such human-driven change anthropogenic evolution, and it is fast because the selection is intense and the generations are short.

The examples share a lesson: evolution has no goal and no foresight. Mutations arise by chance, and which ones spread depends on the environment of the moment. Evolution is a stochastic process, shaped by chance events and by selection, not a planned march towards a target.

6.4 What is Adaptive Radiation?

In 1835 the Beagle spent five weeks at the Galápagos Islands, about 900 km west of Ecuador. Darwin collected small, dull-coloured birds there without realising they were close relatives. Back in London the ornithologist John Gould showed that they formed a group of distinct but related species, each with its own kind of beak. Today we count about 18 species of these Darwin's finches. All descend from one seed-eating ancestor that reached the islands from the American mainland within the last few million years.

Each species is suited to a different way of feeding (Fig. 6.7). Ground finches crack seeds with deep, strong beaks, and the bigger the beak, the harder the seed it can open. Cactus finches probe cactus flowers with long, pointed beaks. Tree finches crush insects, the warbler finch picks tiny insects off leaves with a thin, sharp beak, and the vegetarian finch strips buds and fruit with a thick, curved one. The woodpecker finch even pries grubs from bark with a cactus spine held in its beak.

The evolution of many species from one ancestor, as its descendants spread into different habitats or ways of life, is adaptive radiation (अनुकूली विकिरण). Darwin's finches are a textbook case, because the islands offered many empty niches and few competitors.

Australia shows adaptive radiation on the scale of a continent. Isolated from other land masses for tens of millions of years, it was home to pouched mammals, the marsupials, which spread into almost every mammalian way of life: grazing kangaroos, burrowing wombats, leaf-eating koalas, gliding sugar gliders, the termite-eating numbat, a marsupial mole that 'swims' through sand, and predators such as quolls, the Tasmanian devil and the now-extinct thylacine, or Tasmanian wolf.

Many of these marsupials look and live remarkably like placental mammals on other continents (Table 6.4). The thylacine resembled a wolf, and the sugar glider is almost a copy of a flying squirrel. The two groups are only distantly related, since their last common ancestor lived more than 160 million years ago, but similar ways of life selected similar body forms. This is convergent evolution: two separate adaptive radiations, on different continents, arriving at matching solutions.

6.5 Biological Evolution

Natural selection needs three ingredients: variation, inheritance and unequal reproductive success. The first cells already had all three, since some would have used the chemicals around them more efficiently than others, so selection is as old as cellular life itself.

How fast it works depends on generation time. A bacterium can divide in about 20–30 minutes, so a culture passes through dozens of generations a day, while an elephant takes well over a decade to produce its first calf. Since 1988 the biologist Richard Lenski and his team have kept twelve flasks of E. coli evolving on a diet of glucose. Their medium also contains citrate, which ordinary E. coli cannot use when oxygen is present. After about 31,500 generations one population evolved the ability to feed on citrate and boomed. A change of that size would take a mammal hundreds of thousands of years; in the bacteria it took about fifteen.

Only differences written in the genes can be passed on, so only inherited traits can be selected. A bacterium that survives a drug only because it sat in a corner the drug never reached passes on nothing useful. Being adapted, that is well suited to survive and reproduce in a particular environment, is the result of many rounds of selection on inherited differences.

Put together, Darwin's theory stands on two pillars: branching descent, in which all species are related through a tree of common ancestors (Fig. 6.3), and natural selection, the main process that fits each lineage to its surroundings.

Why does a giraffe have such a long neck? Half a century before Darwin's book, in 1809, Jean-Baptiste Lamarck gave an answer built on two rules. Organs that are used a lot grow, while organs left idle wither (use and disuse). And whatever an animal gains or loses this way during its life is handed on to its young (inheritance of acquired characters). On this view each generation of giraffes, stretching for the highest leaves, added a little neck length that its calves inherited.

The idea fails because changes acquired during life do not reach the eggs and sperm. In the 1880s August Weismann cut off the tails of mice over several generations, and every litter was still born with normal tails. A wrestler's thickened ears or a weightlifter's muscles are not inherited for the same reason. The Darwinian account of the giraffe is different: calves born with slightly longer necks reached more food in dry seasons, survived better and left more calves of their own.

One more ingredient came from economics. In 1798 Thomas Malthus argued that human populations tend to grow faster than their food supply. Darwin read Malthus in 1838 and saw that the argument applies to all of nature. His reasoning can be laid out as five observations and three inferences (Fig. 6.8).

Put the two halves together and selection becomes unavoidable. When not every offspring can survive, the ones whose inherited traits suit their surroundings will, on average, become the parents of the next generation. Repeat that over thousands of generations and the average character of the population shifts in the direction the environment favours. That is why Darwin's book is titled On the Origin of Species by Means of Natural Selection: the same everyday sorting, accumulated over geological time, can produce new species.

6.6 Mechanism of Evolution

Darwin's theory had a gap: he could not say where variation comes from or how it is inherited. Mendel's work on inherited 'factors' (1866), the genes of Chapter 4, lay unnoticed until 1900, and Darwin never knew of it. Around the same time, in fields near Amsterdam, the Dutch botanist Hugo de Vries found evening primroses (Oenothera lamarckiana) strikingly unlike their neighbours, taller or broader-leaved, which bred true. He named such sudden heritable changes mutations (उत्परिवर्तन).

From these plants de Vries built his mutation theory (1901): a new species appears suddenly, in one large leap, which he called saltation. Darwin had pictured slow piling-up of small differences sorted by selection; de Vries made the jumps themselves the engine of evolution. Ironically, most of his primrose 'mutants' turned out to come from unusual chromosome behaviour, such as extra sets of chromosomes, rather than from changes in single genes.

The two views were reconciled in the 1930s and 1940s by population genetics. R. A. Fisher, J. B. S. Haldane and Sewall Wright showed mathematically that small heritable differences, sorted by selection, can change a population quickly, and Theodosius Dobzhansky and Ernst Mayr linked this to species in the wild. The result, the modern synthesis, holds that mutation and recombination supply variation while selection, drift and gene flow shape it. Haldane later settled in India, became an Indian citizen and continued his research in Kolkata and Bhubaneswar.

A central point of the synthesis is that mutations are random with respect to need: an organism cannot produce the mutation its environment calls for. Joshua and Esther Lederberg showed this neatly in 1952 (Fig. 6.9). They pressed a velvet pad onto a plate of bacterial colonies and printed the same pattern onto plates containing the antibiotic streptomycin. Resistant colonies grew at the same positions on every copy, so the resistant cells had been present on the original plate before they ever met the drug.

6.7 Hardy–Weinberg Principle

Seen from genetics, evolution is a change in the genetic make-up of a population over generations. To detect such a change we need a baseline: what happens to a population's genes when nothing is pushing them? That baseline is the Hardy–Weinberg principle, worked out independently in 1908 by the English mathematician G. H. Hardy and the German physician Wilhelm Weinberg.

Gene pool and allele frequencies

Imagine every copy of every gene carried by every member of a population as one shared reservoir: the population's gene pool. For one gene with two alleles, $A$ and $a$, a diploid population of $N$ individuals holds $2N$ copies. The fraction of those copies that are $A$ is the allele frequency of $A$, written $p$, and the fraction that are $a$ is $q$. With only two alleles, every copy is one or the other.

In a randomly mating population, each offspring is formed from two gametes drawn at random from the gene pool. The chance that an egg carries $A$ is $p$ and the chance that a sperm carries $A$ is also $p$, so the chance of an $AA$ offspring is $p\times p=p^2$. The derivation below completes the argument (Fig. 6.10).

A population that meets these conditions is in Hardy–Weinberg (genetic) equilibrium: its gene pool does not change. The principle is useful precisely because real populations rarely meet every condition, so a departure from it is a sign that evolution is under way.

What disturbs the equilibrium

If measured genotype frequencies differ from $p^2$, $2pq$ and $q^2$, or allele frequencies change between generations, something is acting on the population, and the size and direction of the difference hint at what. Five forces are usually listed as able to push a population away from equilibrium:

1. Gene flow (gene migration): pollen blown between fields, or animals moving between herds, carry alleles from one gene pool into another. When this happens again and again, the populations involved stay genetically similar.
2. Genetic drift (आनुवंशिक विचलन): allele frequencies change by chance because only some individuals happen to breed. Drift is strong in small populations and negligible in very large ones.
3. Mutation: creates new alleles. It is the ultimate source of variation but, on its own, changes frequencies very slowly.
4. Genetic recombination: crossing over and independent assortment during gamete formation reshuffle alleles into new combinations, giving selection fresh variation to act on.
5. Natural selection: genotypes that survive and reproduce better contribute more alleles to the next generation.

Non-random mating, such as mating between relatives, also upsets the equilibrium: it raises the share of homozygotes without, by itself, changing allele frequencies.

Drift is most dramatic when a population passes through only a few individuals. If a small group leaves to found a new population, on an island for example, its gene pool is a small chance sample of the old one: some alleles become over-represented and others are missing altogether. This is the founder effect (Fig. 6.13). A population that crashes to a few survivors and then recovers goes through a similar bottleneck.

India has striking examples. Asiatic lions in Gir shrank to a few dozen animals in the early twentieth century; they now number in the hundreds but carry very little genetic variation. Cheetahs, recently reintroduced to Kuno National Park in Madhya Pradesh, went through ancient bottlenecks and are so uniform that skin grafts between unrelated cheetahs are often accepted. If a founder population is also isolated, drift and selection can carry it so far that it becomes a new species.

Three ways selection shapes a trait

Many traits, such as body size, beak depth and birth weight, vary continuously, so the values in a population form a bell-shaped curve. Selection can act on such a curve in three ways (Fig. 6.15). In each case it changes allele frequencies in the gene pool, which is why it counts as evolution in the Hardy–Weinberg sense.

• Stabilising selection favours the average and removes both extremes, so the curve becomes taller and narrower while its mean stays put. Human birth weight was long a classic case: babies much lighter or much heavier than average were less likely to survive.
• Directional selection favours one extreme, so the whole curve shifts that way. After a severe drought in 1977 on the Galápagos island of Daphne Major, mainly large, hard seeds were left; medium ground finches with deeper beaks survived, and their offspring had measurably deeper beaks.
• Disruptive selection favours both extremes over the middle, so the curve can split into two peaks. Black-bellied seedcrackers in Cameroon have either small or large bills, suited to soft or hard seeds; birds with in-between bills handle neither well.

6.8 A Brief Account of Evolution

Deep time is hard to picture, so squeeze the Earth's 4.54 billion years into a single 24-hour day. The first cells appear at about 4:30 in the morning. Oxygen from photosynthesis builds up in the air at about 11:20. The first complex (eukaryotic) cells arrive in the afternoon. Animals with shells and skeletons appear only after 9 p.m., and dinosaurs rule from about 10:45 until 11:39 p.m. Our species arrives about six seconds before midnight; farming, a fifth of a second before.

For most of that day life was microscopic, and its great invention was oxygen-releasing photosynthesis. Microbes like today's cyanobacteria began using sunlight to pull electrons out of water, the reaction you studied in Class 11 as the light reaction of photosynthesis, and gave off oxygen as a by-product. At first the oxygen rusted the iron dissolved in the seas, laying down layered iron-rich rocks called banded iron formations; much of the iron ore of Odisha and Jharkhand comes from rocks of this kind. Only later did it build up in the air, making an ozone shield and oxygen-hungry cells possible. Life with many cells came later still.

Around 539 million years ago, at the start of the Cambrian period, animals with shells, legs and eyes appear abruptly in the fossil record, and almost every major invertebrate group is present within a few tens of millions of years. The first vertebrates, jawless fish, follow by about 520 million years ago; fish with jaws come later. Plants reached land before animals did: small moss-like plants by about 470 million years ago, followed by vascular plants with stiff stems and roots (Fig. 6.16). When animals crawled ashore, a green land was waiting to feed and shelter them.

One group of fish, the lobe-fins, had muscular fins supported by bones. In shallow, oxygen-poor waters about 375 million years ago, lobe-fins able to prop themselves up and gulp air gave rise to the first four-legged animals, ancestors of amphibians such as frogs and salamanders. Lobe-finned fish of the coelacanth group were thought to have died out with the dinosaurs, until 1938.

Amphibians must return to water to lay their eggs. About 312 million years ago reptiles evolved a tougher solution, the shelled (amniotic) egg, whose membranes keep the embryo moist on dry land. Meanwhile, in the swampy forests of the Carboniferous period, giant club mosses and tree ferns (pteridophytes) grew, died and were buried, slowly turning into the coal mined today. After the greatest extinction of all, 252 million years ago, reptiles diversified on land, in the sea (the fish-shaped ichthyosaurs, marine reptiles rather than dinosaurs) and in the air (pterosaurs).

Dinosaurs appeared about 230 million years ago, lived for some 165 million years and ruled the land for most of that time. Tyrannosaurus rex, one of the last, was about 12 m long with teeth the size of bananas. Then, 66 million years ago, an asteroid about 10 km wide struck what is now Mexico. The impact, together with immense volcanic eruptions in India that built the Deccan Traps at about the same time, caused a sudden climate catastrophe, and about three-quarters of all species vanished, including every non-flying dinosaur. One group of small feathered dinosaurs survived: the birds. Crocodiles, lizards, snakes and turtles are the other reptile lines that came through.

Mammals are almost as old as dinosaurs. The earliest, about 205 million years ago, were shrew-sized insect-eaters active at night, and for more than 100 million years mammals stayed small. The asteroid changed that. Within 10–15 million years of the impact, mammals with fur, milk-fed young and keen senses had moved into the empty niches as grazers, browsers, predators and bats; most species came to carry their young inside the mother until birth.

Moving continents decided where each group flourished. South America drifted alone for tens of millions of years and evolved its own cast: hoofed litopterns that looked like horses, toxodonts built like hippos, giant ground sloths and armoured glyptodonts the size of small cars. About 3 million years ago a land bridge formed at Panama; cats, dogs, bears and deer spread south, and most of the old South American groups died out. Australia, in contrast, has been an island continent ever since it parted from Antarctica, so its marsupials met no placental rivals until people and their animals arrived (Section 6.4).

Some mammals went back to the sea. Whales and dolphins descend from four-legged, hoofed land mammals, seals from bear-like carnivores, and dugongs and manatees (sea cows) from relatives of elephants. India still has dugongs in the Gulf of Mannar and around the Andaman Islands. Horses, elephants and dogs also have well-documented fossil histories worth exploring on your own; Exercise 10 traces the whales.

6.9 Origin and Evolution of Man

Around 15 million years ago many kinds of apes lived in the forests of Africa, Europe and Asia. Your textbook names two of them. Dryopithecus was a European ape that climbed through trees. Ramapithecus was described in 1934 from jaws found in the Siwalik Hills of India and was long thought to be an early human ancestor, but better fossils later showed that it belongs with Sivapithecus, a relative of today's orangutans. Both lived long before the human line split from the chimpanzee line, an event that DNA and fossils place at about 6–7 million years ago.

Members of the human branch after that split are called hominins (your textbook uses the older word hominids). The oldest, such as Sahelanthropus from Chad, about 7 million years old, are known from only a few bones. Between about 4 and 3 million years ago, the australopithecines ('southern apes') walked upright in East Africa while still climbing trees. The skeleton nicknamed Lucy, from Ethiopia, is 3.2 million years old, and footprints pressed into volcanic ash at Laetoli in Tanzania show upright walkers 3.66 million years ago. They stood about 1–1.5 m tall, had brains of about 400–500 cc, much like a chimpanzee's, and ate mostly plant food.

About 2.4 million years ago Homo habilis ('handy man') appears in East Africa, with a somewhat larger brain of about 550–700 cc and simple stone tools made by knocking flakes off pebbles (Fig. 6.17). Cut marks on animal bones from this time show that early Homo used such tools to get at meat and marrow, probably by scavenging as well as gathering plants.

Homo erectus ('upright man') appears about 1.9 million years ago, with a body much like ours, a brain of roughly 600–1100 cc (about 900 cc on average) and carefully shaped hand-axes. It ate meat regularly, probably used fire, and was the first hominin to spread out of Africa, reaching as far as China and Java, where the first fossils, nicknamed Java Man, were dug up in 1891. In India, a fossil skull-cap found in 1982 at Hathnora on the Narmada river in Madhya Pradesh belongs to an archaic human of broadly this stage.

Neanderthals (Homo neanderthalensis) lived in Europe and western and central Asia from about 400,000 to 40,000 years ago. Their brains were as large as ours or larger, about 1200–1750 cc. Neanderthals buried their dead, looked after injured companions, made fine stone tools and dressed in animal hides through the ice-age winters.

Our own species, Homo sapiens, originated in Africa: the oldest fossils, from Jebel Irhoud in Morocco, are about 300,000 years old. Groups of modern humans spread out of Africa from roughly 70,000–50,000 years ago, reaching India, Australia and Europe and, much later, the Americas. On the way they met and interbred with Neanderthals, so about 1–2 % of the DNA of most people whose ancestry lies outside Africa is Neanderthal. All living humans belong to one species with little genetic difference between populations; most human genetic variation is found within any one group rather than between groups.

Art, ornaments and long-distance trade spread with modern humans. The oldest known cave paintings, on the island of Sulawesi in Indonesia, are more than 45,000 years old. In India, the rock shelters of Bhimbetka in Raisen district, Madhya Pradesh, a UNESCO World Heritage Site, carry paintings of animals, hunts and dances, the oldest at least 10,000 years old. Farming began around 11,000–12,000 years ago in western Asia as the last ice age ended, and by about 9000 years ago people at Mehrgarh, at the foot of the Bolan Pass, grew wheat and barley and herded cattle. After farming came villages, towns and cities; from that point on, the record of our species is read mostly by archaeologists and historians rather than by palaeontologists.