v1.2.2 / chapter 12 of 28 / 01 aug 08 / greg goebel / public domain
* A bit less than a quarter of a billion years ago, following a global catastrophe that wiped out a good portion of life on Earth, reptiles became the dominant large organisms on the planet, in the form of the "dinosaurs", as well as marine and flying species. Only a few tens of millions of years ago, another catastrophe displaced the dinosaurs, leading to the modern dominance of birds and mammals as large species. One family of mammals, the hominids, that emerged recently is of particular interest to us, since we are among its members.
* The Triassic and Jurassic periods, beginning 240 million and 205 million years ago respectively, were the first two of the three great ages of reptiles. Two major families of "dinosaurs", both diapsids, would become established during the Triassic:
It should be noted that, unlike modern crocodiles or lizards, the hadrosaurs and carnosaurs walked on two legs and so weren't likely to have had any problem running and breathing at the same time.
It is a popular misunderstanding that all the exotic reptiles of this era were dinosaurs, but there were other families of diapsids as well:
In modern times, the dinosaurs along with the pterosaurs and crocodilians are referred to as the "archosauria"; the archosaurs along with the lizards are called the "sauria"; with the sauria, the icthyosaurs, and the plesiosaurs and kin completing the diapsids.
While the scenery might have been dominated by reptiles, they weren't the only creatures in the game: "ammonites", shelled cephalopods, were common, as well as frogs and salamanders. Birds, in the form of the Archaeopteryx, as well as primitive mammals -- creatures not so different from modern shrews -- emerged in the late Triassic.
Incidentally, while not all birds fly, they are generally seen as flying creatures, and how they acquired that trick is something of a mystery. While there was a long-standing assumption that they evolved out of gliding animals, some have persuasively argued that this is an evolutionary path unlikely to lead to true flight. The gliders tend to be more reminiscent of parachutes than aircraft, and it is a bit hard to think of a parachute as a direct stepping-stone to an aircraft -- man-made parachutes were around for about a century before humans learned to properly fly. In addition, the changes involved in the transition from a glider to a flapping-wing flier are roughly on the same order of elaboration as those involved in turning a flightless animal into a flapping-wing flier.
A subtler theory suggests that they began as running animals capable of boosting speed or taking to the air every now and then to escape predators, along the lines of the way a quail will run for as long as it can, taking to the air in a burst of energy when pursuit gets too hot. That sort of evolutionary "logic" can be seen in the flying fish, with its aerial leaps from the waves to escape pursuit. Flying fish could never really go on to long-distance flight, however, since they can't live out of the water for long, and the adaptations required for them to do so were just too far outside the evolutionary envelope.
* As mentioned previously, in modern times birds are classified among the reptiles, and in fact they are now formally described as dinosaurs. This notion was controversial for decades, it sounded too ridiculous, but it was obvious as far back as the days of Thomas Huxley that birds were an offshoot of the dinosaurs, and under the rules of cladistics admitting they were dinosaurs ultimately became unavoidable. Taxonomy is all about determining family relationships, and if there was to be a family of dinosaurs, it had to include all the members of the family on the family tree. It wasn't arbitrary to include the birds: it was arbitrary to exclude them.
Birds are descendants of small saurischian carnosaurs, and the problem with excluding the birds from the dinosaur group is that they are far more closely related to saurischians than saurischians are to ornithischians. There is no way to exclude the birds from the dinosaurs without driving a taxonomic wedge between the saurischians and ornithischians. Birds are more closely related to the allosaurus than the allosaurus is to the stegosaurus -- despite the similarity in hip structure, birds are not closely related to the ornithischian dinosaurs. To make the birds even more difficult to extract from the dinosaurs, they are more closely related to the carnosaurs than the carnosaurs are to the big sauropods.
Early birds like the toothed Archaeopteryx are so similar to the small carnosaurs known as "maniraptors", that if there weren't impressions of feathers and wings in the fossils, they would likely be classified as them, and recent fossil finds show that some of the small carnosaurs had feathers as well. The general modern assumption is that feathers preceded birds, being an adaptation that came in handy as insulation in climates a bit too cool for scaly reptiles, and later acquired roles in camouflage, sexual display, and flight. However, this is an assumption, and it is perfectly possible that at least some of these maniraptors were actually flightless birds, descendants of flying ancestors, and not really part of an ancestral line to the birds. Such are the ambiguities lurking in the fossil record.
It was once thought that feathers were derived from reptilian scales, but nobody believes this any more. The proteins that make up feathers differ from those that make up scales, and feathers originate from a subdermal layer while scales grow on the surface of the skin. Unfortunately there's no fossil evidence for intermediate feather forms -- Archaeopteryx had fully formed feathers. This is not too surprising because feathers are only evident in the most detailed fossils, but it would be nice to learn more. Teeth were common among early birds and were retained for a long time, so it's a bit surprising that absolutely no modern birds have teeth. They do retain the gene for teeth, however, and that gene can be used to produce teeth like those of ancient birds in lab experiments.
* Some critics of evolutionary theory deny that Archaeopteryx is an intermediate form to modern birds, apparently because it was fully feathered and had the rough general appearance of a modern bird. However, Archaeopteryx is structurally far more a classic dinosaur than bird. As noted it had teeth, claws on the wings, an undeveloped breast structure that limited its flying ability -- and it also a long bony reptilian tail. Given an image of a maniraptor skeleton, an Archaeopteryx skeleton, and a pigeon skeleton, an untrained observer would group the first two skeletons together, and clearly see the pigeon -- with its stubby tail and elaborate breastbone structure -- as a different creature. A trained observer would confirm and reinforce the same grouping.
The critics insist that a true "transitional form" would be intermediate in all respects, for example with "intermediate" feathers, but this is a fantasy. In practice one feature of an organism may evolve in a different sequence and rate than another, a notion referred to as "mosaic evolution". There is no law or logic requiring that, say, all the features we recognize as birdlike had to evolve tidily in parallel. As noted, it is widely suspected that dinosaurs developed feathers to begin with, and feathers were simply "handed down" to the bird descendants.
The critics also like to insist that Archaeopteryx was not really the ancestor of modern birds. It actually may not have been, but this is a minor technicality. Darwinian evolution defines a branching tree of lines of descent, and there's no saying if Archaeopteryx was on the direct line of descent to modern birds or if it was off on some side-branch that eventually died out. Does this make a significant difference? Not really, since even if Archaeopteryx wasn't on the direct line of descent to modern birds -- and it has no features that rule it out -- it was a brother to the species that was, and it is a reasonable inference to assume the two sibling species had enough features in common to allow Archaeopteryx to serve as a general template for what early birds looked like.
* In modern times, birds consist of two groups, including the "ratites" -- ostriches, emus, rheas, the extinct moas of New Zealand and elephant birds of Madagascar, and some others -- and everything else. The ancestors of the ratite branch clearly go back to the age of dinosaurs, since they are effective globally distributed even though few ratites can fly or, as far as the fossil record goes, have been able to fly for a long time. Their origins trace back to the days before the breakup of Pangea.
Relics of the early days of mammals survive, in the form of the "monotremes" of New Zealand like the platypus that lay eggs but give milk to their young. In the case of the platypus, however, except for its reptile-like inclination to lay eggs it cannot be considered a "living fossil" in any way, instead being an animal with unusual and elegant adaptations, sporting webbed feet, an ultrasensitive toothless "duckbill" for hunting worms and the like in dark brackish water, and (for the males) venom spurs on the hind feet. It is the only truly venomous mammal. It was once seen as made up from the spare parts of other animals as if it were some sort of monster created by a mad scientist, but it is as suited to the niche it occupies as, say, whales are to theirs.
The age of dinosaurs also saw the introduction of the other two surviving branches of the family of mammals, the "marsupials" like the kangaroo and opossom -- which give birth to young in a near-embryonic state after a short and convenient gestation period -- and the "placental" mammals, which give birth to generally developed young after a long gestation period, the offspring being linked to the mother in the womb by a link known as a "placenta". It should be noted that mammals are classified as "amniotes" even though only the monotremes lay eggs, because all the mammals are descendants of amniote egg-layers -- in the same way that a snake is classified as a "tetrapod".
The most common mammals of the era were actually the "multituberculates", mostly rodent-like creatures, all now vanished, distinct from the marsupials but with much in common with them. The mammals would remain "bit players" for aeons, with no known mammal fossil from the age of dinosaurs being bigger than a cat.
* The Cretaceous period, beginning 135 million years ago, was the last of the great ages of dinosaurs. In terms of the beasts of the lands, the Cretaceous was different in degree but not in kind, with the carnosaurs reaching their peak in the fearsome "tyrannosaurus" and in modern times acquiring some movie-monster notoriety through the introduction of the quick-footed "velociraptor". The pterosaurs also reached a peak of sorts, represented by the huge "quexactolus", the size of a sailplane. There were sea turtles twice the dimensions of modern sea turtles.
The real innovation of the Cretaceous was somewhat less imposing: flowering plants, and the pollinating insects such as bees to go along with them. Before the Cretaceous the world had no flowers, with plants generally spreading their pollen to the wind, an inefficient process; by the end of the era, the "angiosperms" would be the rulers of the plant worlds. Schemes also arose for the distribution of seeds, such as berries and fruits -- which would be eaten by birds and the like, with the seeds then excreted some distance from the parent plant -- or cockleburrs and the like, to cling onto beasts and be carried some distance. In addition, the angiosperms developed a wide range of defenses, based on spines or toxins.
The Cretaceous came to an abrupt end 65 million years ago. The general consensus for some years was that a giant meteor impact that hit in what is now the Yucatan peninsula causes a wave of burning, followed by a long winter and a climatic shift. The impact clearly occurred, but there has been considerable debate among geologists and paleontologists whether it was really responsible for the mass extinction. All agree that there was a mass extinction.
* Whatever wiped out the dinosaurs and the other great reptiles, the Tertiary period, beginning 65 million years ago, brought birds and mammals to the fore. There is also discussion over how rapid this transition was, with some claiming the newcomers were getting established well before the end of the Cretaceous. However the transition took place, there was a transition, with all the major mammal orders, mostly placentals, generally represented by about 50 million years ago.
* The "carnivores" include the early "creodonts", and the more modern cats, wolves, bears, weasels, civets, and so on. Extinct forms include the various forms of sabre-toothed cats and the oversized "dire wolf". The name "carnivore" is a bit misleading, since of course there are plenty of non-mammalian carnivores, and not all of the order of carnivores are actually meat-eaters -- bamboo-eating panda bears for example. The seals or "pinnipeds" were once classed as a separate order, but if they were excluded the carnivores wouldn't be a true clade, and so they are now regarded as a family. It seems from genetic evidence that they are most closely related to bears.
* The "ungulates", the hoofed animals, including a diversity of early species once collected under the name of "condylarths", many of which actually looked like small predators, followed by odd-toed ungulates like horses and rhinos ("perissodactyls") and cloven-hoofed / even-toed ungulates like cattle and deer ("artiodactyls"). The two modern groups of ungulates are not actually very closely related, and there were the extinct cloven-hoofed "notoungulates" and odd-toed "litopterms" of South America that were also unrelated, not merely to existing orders of ungulates but to each other. This caused no end of confusion to early paleontologists, since there was a tendency to force them into the existing two groups of ungulates, despite the fact that they didn't seem to be a very good fit.
The oldest ungulate fossils are about 55 million years old, though they had more than two hooves. Since then the extra digits have become increasingly vestigial, though sometimes horses will obtain larger auxiliary digits through genetic throwbacks. There were interesting dead ends among the ungulates, including the "titanotheres", huge perissodactyls with an odd array of horns; the "calicotheres", great robust beasts with hooves become claws; rhinos bigger than elephants; and a giraffe-like camel, the Aepycamelus, once known as Alticamelus.
Believe it or not, whales and dolphins, which were once thought to be derived from the carnivore family, have been seen through their genetic patterns to be, surprisingly, an offshoot of the ungulates. Their closest living relative is the hippopotamus, and in fact a hippo is more closely related to a whale than a hippo is to a cow. For this reason, in modern times the order "cetacea" has been reduced to a family, and the whales have been absorbed into the artiodactyls to form the "cetartiodactyls". It's an awkward thought, but from a taxonomic point of view the alternative is even more awkward. Of course the whales are very distinct from their land-based relatives, and so even though they have been demoted to a family, they are generally described in the literature a distinctive group.
The cetacean family tree is well established in the fossil record from relatively recent finds, with precursor species featuring four fins -- in fact, in modern times rare throwback dolphins with four fins have been occasionally caught, with one captured alive in 2006. Even earlier species, discovered in the 1990s and including a complete fossil, had feet and could clearly get around on land. Some early whales were clearly waders; but the lineage that went to the open sea was the one that became established.
The discovery that whales were so closely related to ungulates was a big shock. That led to some worries about the fact that there are two general classes of whales: the big "baleen" whales, which gulp in big mouthfuls of small food, generally the little shrimplike "krill"; and the toothed whales, like dolphins and killer whales. What if the two subfamilies of whales were unrelated as well? Genetic analysis showed they were actually related, and biologists breathed a sigh of relief. Incidentally, in 1866 Ernst Haeckel published his own read on the tree of life, and astoundingly he not only placed cetaceans as a branch of the artiodactyls, he also placed hippos as more closely related to whales.
* The elephants, including the extinct mastodons and mammoths, form the order "proboscidia" and are an offshoot of the ungulate family, related in a "superorder" named the "afrotheres" to the aquatic manatees and even, it is generally believed, the rodentlike hyraxes. The fact that Haeckel's correct placement of the whales in his tree of life was not much more than an educated lucky guess was demonstrated by the fact that he classified the manatees as a branch off the whale family tree, which they are not.
* The "xenarthans" are represented in modern times by sloths, anteaters, and armadillos. In the not too distant past there were giant ground sloths, as big as elephants, and giant armadillos the size of bears, or "glyptodonts". The pangolins -- scaly anteaters -- of the Old World are different beasts, the sole members of the order "pholidota", more closely related to the carnivores; and the aardvark of South Africa is also an order of its own, a member of the afrothere superorder. All these creatures were once lumped together as "edentates", until it was realized that their external forms were misleading and they didn't form a close-knit family.
* The "rodents" include mice, rats, squirrels, chipmunks, beavers, muskrats, and the like. They have proven a very successful order of mammals, having a solid presence over the globe. The rabbits and hares are a separate but closely related order. Incidentally, there was a discussion in the 1990s over whether guinea pigs were actually rodents and needed their own grouping, but the consensus these days is that they are indeed rodents.
* The classic order of "insectivores" includes the shrews, moles, and hedgehogs. The name "insectivore", or "insect eater", is misleading, since not all of them eat insects, and many other kinds of animals are insect-eaters. It turns out that shrews, moles, and hedgehogs, while related among themselves, are only loosely so, leading to an argument over their classification.
Some other beasts once called insectivores ended up being very distantly related, the most spectacular exception being the long-legged "sengi", sometimes misleadingly known as "elephant shrews" because of their long noses, which also turned out to be afrotheres, more closely related to elephants. Similarly, the tenrecs of Madagascar and parts of Africa, which include shrewlike and hedgehog-like forms, and the golden moles of southern Africa are now each seen as separate orders, being classed among the afrothere superorder.
* The bats or "chiroptera" used to be regarded as a flying offshoot of the insectivores, but it now seems they have different roots. There are two general groups of bats -- small bats, mostly insect-eaters but with some specializations such as pollinators and (infamously) vampires among them; and the big "flying foxes" or "fruit bats". Some place the two groups of bats as separate orders, and the roots of their actual origins are controversial since fossils of bats are rare.
There are also the "colugos" of Southeast Asia, which resemble large flying squirrels but have a reproductive system similar to that of marsupials. Their origins are also controversial, with some claiming a relationship to bats, others a relationship to ...
* The primates, including the lemurs and lorises; the big-eyed tarsiers; the Old World and New World monkeys; and the apes. Some have lumped the squirrel-like "tree shrews" into the primates, but few are now comfortable with that notion. Not being all that much like true shrews, either, they have been stuck in their own order until we can learn different.
* While the Tertiary is often called the "age of mammals", that is something of an oversimplification: it was also the era when birds came of age, as well as flowering plants and, not at all coincidentally, social insects. Most variants of modern birds emerged, with some interesting extinct lineages, such as the "terror birds" of South America, which were oversized flightless predators with great sharp beaks and meathook claws on their stubby wings, amounting to something of a "reinvention" of their carnosaur ancestors. The same rough configuration survives today in substantially smaller forms of ground-living predatory birds, such as the South American crested sereina and the secretary bird of Africa.
* The last few million years of the Tertiary saw the introduction of an interesting subset of the apes: the "hominians", the apes that walked upright, leading to modern humans. Humans were once regarded as a separate family from the apes, but modern genetic analysis showed that chimps and humans are more closely related than chimps and gorillas, making the distinction of humans into a separate family unsupportable, and now they are all known as "hominids", with our own species being the only surviving hominian.
There has been some argument over exactly how close humans are to chimps, with estimates of the similarity ranging from about 94% to over 99%. The precise ratio is somewhat academic, since nobody denies that the genetic resemblance between humans and chimps is greater than the genetic resemblance between humans and any other species. It is generally thought, though absolutely nobody wants to try and find out, that humans and chimps are so similar that they could interbreed, though probably not produce fertile offspring.
Anyway, the earliest of the upright apes led to the "australopithecines", which included a side branch to a giant race known as "robustus", and they seemed to have used primitive stone tools. Another branch led to a more humanlike creature, Homo habilus, sometimes called "early Homo", and then to Homo ergaster, followed by the similar Homo erectus.
By the beginning of the Quaternary period, a mere 1.5 million years ago, the climate was beginning to shift rapidly through a set of ice ages, which was accompanied by the rise of modern human forms. It is generally thought that the Homo erectus line went to a dead end, but another branch off the Homo habilus line led to Homo heidelbergensis, a more recognizably human form, with a brain only somewhat smaller than our own. Homo heidelbergensis led in turn to the Neanderthals, beetle-browed but close enough to modern humans to pass for them, and clearly users of tools and fires; and the modern Homo sapiens, an evolutionary development only a few hundred thousand years old.
About 40,000 years ago, Homo sapiens abruptly left the era of primitive stone tools and began to produce pottery, artworks, paintings, and ever more sophisticated tools, an event some have called the "Great Leap Forward", which has continued to production of symphonies, movies, jumbo jetliners, and Mars probes.
* One of the puzzles in human evolution is the question of why the hominians ended up walking upright. What selection forces pushed the hominians down this branch? Some have suggested it was driven by increased intelligence. That doesn't fly, since the earliest known hominians were fully upright, but didn't have brains any bigger than those of a chimp -- another example of mosaic evolution, with the upright stance not being closely related to brain growth.
Others have suggested that use of tools drove the process, with those individuals that could walk more upright being better tool-users, but in fact the early erect hominids weren't much bigger on using tools than chimps. One plausible idea is that upright hominids were better able to carry food around and stash it; one imaginative idea is that our ancestors liked to forage in the water, where standing upright is obviously more workable than going on all fours.
Some suggest that the hominians picked it up before they ever adapted to life on the ground, since some species of apes, such as gibbons, seem to like to get around on two feet in the trees. Richard Dawkins, always eager to push his meme concept, suggests that one ape in a group of apes simply learned there were advantages to walking upright, such as being able to pick fruit and carry it around, and made more of a habit of it than the others. The advantages became obvious to the others, so they imitated him -- modern chimps are clever creatures, perfectly capable of learning by example, and these early hominians could be expected to be about as bright -- and this tribe of apes then set off on down the selection treadmill, becoming more and more upright over the generations.
Dawkins points out that some chimps in captivity seem to like walking upright more than others, though it doesn't really buy them much in that environment, since they get fed by their keepers the same as others. It should be noted that the pretty little gerenuk antelopes, though clearly four-footed, like to stand on their hind legs to browse on trees and seem to balance themselves fairly well -- would it be too much of a stretch to think of them as possible precursors to upright antelopes of the future?
It is even more of a puzzle as to why the hominians developed such big brains, a tripling of brain size from australopithecines to modern humans. Obviously intelligence provides a selective advantage, but there would be no particular advantage in a brain capable of writing symphonies and designing aircraft just to hunt and gather foods. Felines have enough brains to be skillful hunters and baboons have enough brains to dig up tubers and the like. Would a multiplication in brain size have resulted in much more effective hunting, or acquisition of stockpiles of tubers? It seems hard to believe.
Stephen Jay Gould once suggested that the big brain of the modern human is a neutral feature, a side effect of delayed human maturation, but very few believe this. Compared to body weight, the human brain is huge, far bigger than the relative size of the brain of any other animal, and it imposes significant costs: it makes human birth processes difficult and troublesome, and it also it demands considerable energy, about a fifth of food consumption. If it didn't provide a clear advantage, selection would have worked against it. One long-standing idea is that human tool-making started the cycle towards bigger brains, with more sophisticated tools leading to more sophisticated tool-makers. However, tool-using is not all that unusual among animals -- chimpanzees use twigs to fish termites out of logs -- and so it's hard to argue that tool-using would necessarily boost intelligence. In fact, for most of the time that the hominian brain size was growing, the tools remained little more than primitive broken stones, and tool-making really didn't take off until hominians had big brains.
The more attractive idea is that hominians got big brains because they were in competition with fellow hominians, engaged in an "arms race". Big brains were handy in social interactions, allow the smartest to achieve domination of their social groups. While this notion seems to be on the right track, it has its problems as well, since chimpanzee and baboon societies are highly structured, and there would seem to be a selective advantage in smarter chimps and baboons as well. Why didn't they get on the track to bigger brains? There are some other ideas, discussed later.
* Some critics have suggested that human intelligence is inexplicable in evolutionary terms. As noted above, it is certainly absurd to think that natural selection could have specifically driven the ability to compose symphonies or design aircraft. However, it is straightforward to think that evolution could have driven the development of an increasingly powerful general-purpose biological "computer" that, as also noted earlier, could be "programmed" in unbounded ways -- as demonstrated by the limitless range of skills that different humans can acquire.
Of course, though the critics don't seem inclined to make much of the matter, that range of skills includes the clever and perverse. If we need a better explanation than Darwinian evolution to account for the capability to write symphonies, then we also need a better explanation to account for the capability to pull off ingenious con games, write books crammed with elaborate arguments proving America never landed on the Moon, or design imaginatively disgusting pornographic toys.
It should be added that in the early days of computer programming, artificial intelligence researchers were very optimistic about the progress of their field because it proved relatively straightforward to write computer programs that could, for example, perform algebraic analysis or play chess. The optimism faded away when it proved far more difficult to, say, program a robot to sensibly navigate around a house, something even a cat does easily, or recognize individual humans, a feat that presents no particular difficulty to dogs.
It turns out that supposedly "higher" brain functions can demand far less computing horsepower to duplicate than supposedly "mundane" functions. The cheapest personal computer can play a game of chess that can as a fair bet beat any human player below a competition level of skill; building a vehicle that can reliably navigate on its own is far more difficult. Every set of exploratory actions that a contemporary Mars rover robot performs has to be individually programmed, then run through validation to give some assurance that it will work and not mindlessly drive the rover into a trap.
Certainly a cat can't conceivably play chess. However, it is clear that the enhancement of brainpower in humans over the past few million years rests solidly on the formidable development of brainpower over hundreds of millions of years before that, from the level of the flatworm brain to the level of the cat brain. If our minds see much farther than those of other creatures, we stand high on the shoulders of the minds of those creatures who were our ancestors.
* In any case, modern humans proved to be one of the most successful large animal species to ever arise, gradually spreading over the entire planet. This was not an unmixed blessing. The spread of humans around the globe seems to have been followed by the extermination of large birds and mammals. They were relatively easy prey for hominians armed with effective weapons; once the big animals got scarce, humans turned to smaller species -- rabbits and smaller antelopes -- that bred fast enough to make them harder to kill off completely.
The romantic notion that primitive societies lived in a tidy "balance of nature" doesn't get much respect these days. Richard Dawkins likes to lampoon the popular idea of the "balance of nature" as a Panglossian read on a rougher reality, pointing out that in Darwinian terms nature is competitive, with different species evolving to get the upper hand. When there's a balance, it's more like a "balance of power" than natural harmony, since the system has tendencies toward instability. Upset the balance of power, extinctions result. The bottom line is that the Earth had never seen another species like humans before, capable of harnessing technologies, and almost from the outset humans upset the status quo.
This is not to shrug and claim "extinction is OK." The impact of humans on the environment is so profound, in terms of their often negative effect on other species, that in the geological record the rise of humans may be the cause of yet another era of mass extinctions. Some have worried that human mismanagement of the planet may lead to the extinction of humanity as well. As some cynics have said: "It remains to be proven if there is really any long-term advantage to intelligence."
