(continued) Chapter 3. Long Evolution: Life Emerging
Scaling up from one cell to many may have been a small step rather than a giant leap for early life. A single-celled organism closely related to animals controls its life cycle using a molecular toolkit much like the one animals use to give their cells different roles. “Animals are regarded as this very special branch, as in, there had to be so many innovations to be an animal,” says David Booth, a biologist at the university of California, Berkeley. But… “a lot of the machinery was there millions of years before animals evolved.” Multicellular organisms send messages between their cells and direct the cells to particular roles. The Capasaspora owczarzaki amoeba uses many of the same tricks to switch its single-celled body between different life stages. … In animals, proteins in different organs in the same individual show similar changes.
Science News, November 12, 2016
Leaving all those other types of animals in the dark now, continuing to evolve on their own, on their many separate paths, we’re going to continue exploring out just one of the other paths, the one that leads to our kind, the vertebrates – animals with backbones.
This branch of the animalia is distinguished by having a fully developed segmented bony spine which has a hole down the center (“fully developed” is relevant because some not-fully-vertebrate creatures possess only a one-piece cartilaginous structure which functions similar to a spine). Starting at about upper neck level, the brain’s tail, known as 1) the brain stem and then 2) the trailing spinal “chord,” differentiates itself into a bundle of major trunk nerves which pass down through the center of the hollow spine. The trunk nerves branch off to right and left between each pair of vertebras, each trunk subdividing into ever-smaller branching nerves which connect with, ultimately, every last cell in the body – exactly like the complex wiring which connects thousand of homes back to a power plant. Lending perspective, the adult human body is made of trillions of cells, each and every one of which has hard-wire nerve connection directly back to the brain.
Lending more perspective, the branching shape seen in living nerve systems and electrical power grids are also seen in the way trees are shaped from trunk to upper twigs, in aerial views of river drainage systems and deltas, and in vertebrates’ blood circulatory systems. The branching pattern is indeed ubiquitous in nature, of which we vertebrates and our branching nervous systems are integrally a part, as well as in manmade cultural artifacts such as road networks (interstate to one-lane road) and product distribution systems.
With the vertebrates we have reached an important milestone in our ascent up through the layers and branching patterns of animals which long ago diverged from the plant kingdom and gradually subdivided (speciated) into millions of different kinds of animals. Settling for just dabs of information about all those other vertebrates that aren’t us, we’ll continue moving over them as briefly as possible, saving the best for last.
Types of vertebrates
Vertebrates comprise five broad groups: fishes, amphibians, reptiles, birds, and mammals. A long history stretches behind each of these, originating shortly after the Cambrian period began 500 million years ago – though, I remind you, this is only one-seventh as long a history as that of the bacteria and archaea, compared to whom we vertebrates are mere newcomers.
Interestingly, the oldest vertebrate species lacked jaws – they had to be quick and bully if they wanted to eat. Some of their descendents, beauties like the living hagfish and lamprey eels, still carry the trait from that long ago time. Hinged jaws, which some think a useful innovation, evolved a mere 400 million years ago. Though far fewer in number than the older pre-Cambrian species, our fellow vertebrates are more intimately familiar to us than all those tiny oddballs listed above which mostly live in inaccessible places and can’t be seen with the naked eye anyway. Vertebrates by contrast are easy to find, easy to coexist with, easy to see. And we see that, jaws or not, most of them have four legs.
The word tetrapod means “four-legged,”and this subset of the vertebrates deserves a few prefatory words. Excepting only the fish family for obvious reasons, tetrapods encompass the other four vertebrate families. Birds – over the ages since long-armed dinosaurs learned the advantages of feather-assisted hops, then full flight – have evolved their elongated front legs into highly functional wings, but they’re still tetrapods. Over those same ages whales and other mammals that evolved back into ocean dwellers transformed their four legs into flippers and tail fins, but they’re still tetrapods. The mammals that stayed on land, which is most of us, got by very nicely with their four legs – a very few eventually learning to stand up and walk on two instead of all four. Snakes retain four tiny legs in their skeletons, so they’re tetrapods even though they’ve evolved other means of locomotion. All turtles retain four legs, though those living on dry land have big squatty legs to carry their weight while the legs of those living in ocean habitats are evolving into shapes more like flippers the better with which to swim.
Only the poor fish never had legs, though even here a few species have learned to use their fins for “walking” quite considerable distances across dry land to the next waterhole. Perhaps in time these particular fish will evolve full legs and fully air-breathing lungs, then complete their transition to full time dry-land living.
The extinct vertebrates – dinosaurs
Vertebrates both extinct and extant help us understand the Long Evolution that began with the big bang. Accordingly, our exploration should be prefaced with a few words on one of the early vertebrate groups, those mostly (but not completely) extinct reptiles called dinosaurs. Among the most successful animals that ever lived, dinosaurs enjoyed a 160-million-year run. To give that some perspective, archeological and genetic evidence indicates we humans have been around for about 200,000 years, which is only 0.00125 percent as long as dinosaurs were around. Their success stumbled when most of them went extinct 65 million years ago after a large meteor struck the Yucatan area. In their extraordinary range of shapes, sizes and ways of surviving and reproducing, dinosaurs were as diverse as are modern mammals. We know nothing about their consciousness and can only make wild guesses, of which a few scientists are not inhibited. The few dinosaurs that managed to not go extinct are today called “crocodiles” and “birds.”
Non-extinct vertebrates whose world we share
Our exploration of modern vertebrates begins with the only non-tetrapod. Fishes are classified in three groups, i.e., 1) cartilaginous fishes (sharks, skates and rays), whose skeleton consists of tough flexible cartilage instead of bone, and true bony fishes which comprise two subgroups, 2) ray-finned fishes and 3) lobe-finned fishes. Bony fishes, with nearly 30,000 species, are the most diverse vertebrate group living today. They use gilla to breath, a swim bladder to maintain buoyancy, and have color vision.
Cartilaginous fish (sharks, skates and rays): First appearing in ancient oceans some 450 million years ago, cartilaginous fish today range from fearsome predators (great while sharks, which do indeed have jaws) to harmless mollusc-eaters. Lacking true bone, their skeletons are made of flexible cartilage similar to that in human noses and ears. Even the calcified cartilage of their teeth differs from true bone. A number of shark and ray species are fished commercially or for sport.
Many sharks are fished commercially for their fins only (shark fin soup is very popular in China). This apex predator is caught, the fins are quickly cut off and the shark is thrown back in to die a slow and painful death. The fin itself, undeservedly considered a delicacy despite having virtually no flavour, is used mainly to add texture. Fins are often doused in hydrogen peroxide to make their color more appealing. Claims that shark fin soup have health benefits are not supported by evidence. In fact barely fit for human consumption, all shark tissue has very high levels of mercury and the U.S. Environmental Protection Agency advises women and young children to avoid it. Sharks breed infrequently and some can take a decade or more to reproduce. Thus vulnerable to overfishing, shark decline is evident in every ocean around the globe. A 2007 study found that overfishing of large sharks led to a population boom of the cownose ray, which in turn decimated commercially valuable scallop populations off the North Carolina coast.
Ray-finned fishes: Ray-finned fishes comprise almost 99 percent of the 30,000 bony species, making them the most numerous class of all vertebrates. They are so called because their fins consist of webs of skin attached to and supported by bony or horny spines which attach directly to the skeleton. They are ubiquitous on the earth from the remotest high mountain streams to every depth in the oceans. Ray-finned fishes range from tiny Paedocypris (three-tenths inch long) to long-bodied oarfish (up to thirty-six feet) and massive ocean sunfish (more than five thousand pounds). Most ray-finned species have separate sexes in which reproduction occurs by the female spawning eggs into a nest area over which the male then hovers to release inseminating sperm. A few species are hermaphroditic, starting life as females which sooner or later respond to certain environmental triggers by turning into males which then, male-like, make sure the species gets reproduced. Some fish think this procedure outrageous but tolerance for chance coincidence of birth is growing among them. Whichever scheme they use, in 80 percent of species both parents then leave and the babes are on their own, a custom envied by more complex vertebrates. In the remainder, male care for the young is more common than female care – a custom that has not much caught on among mammals.
If you like to go fishing you will easily recognize certain popular ray-finned game fish – bass, crappie, bluegill, trout, perch and so on.
Lobe-finned fishes: The fins of lobe-finned fish, unique among bony fishes, are stout appendages consisting of many bones and muscles. Among these species are those which, as mentioned earlier, use these flexible lobed fins for walking on land. Of interest in the progression of our evolutionary tale, some scientists think the earliest tetrapods (four legs) may have evolved out of these unusual fishes. We shall let two prominent lobe-finned members, because of their interesting oddness, stand in as representatives for all the lobe-finned fishes.
1) Lungfish are freshwater fish found only in the southern continents, usually in still or slow flowing pools. They may live as long as 80 years. To survive seasonal heat and drought in pursuit of such longevity, lungfish in Africa and South America burrow into mud and “aestivate” (similar to hibernation, whereby a lungfish slows its metabolism to as little as one-sixtieth its normal rate). The Australian lungfish – with a single lung compared to the usual two – is carnivorous and uses electro-reception to locate prey such as small fish, snails, shrimp, worms, tadpoles and frogs. The Australian species can uniquely survive the continent’s extreme dry periods by surfacing and using its gills to breathe air – thereby adding weight to the tetrapod-evolution theory.
2) Coelacanths are two species, both endangered even though they live most remotely in the deep oceans off Africa and Indonesia at depths of two thousand feet or more. Makes you wonder. They can grow to over six feet in length and are thought to live sixty years or more. Possessed of thick scales found otherwise only on extinct fish, this “living fossil” was indeed thought to have gone extinct along with the dinosaurs 65 million years ago until a healthy live one was caught by a South African fishing trawler in 1938. The coelacanth may be called truly weird. Its pairs of lobe fins extend from the body and move in an alternating pattern much the same as legs. A gel-filled cavity in the snout senses electrical currents in order to detect prey in the dark. A hinged skull joint enables the fish to widen its jawless mouth to accommodate large prey. Its tiny heart is basically a straight tube. Ninety-eight percent of its braincase is filled with fat, the rest is brain. And with an oil-filled tube (a “notochord”) serving as a backbone, the coelacanth barely qualifies as a vertebrate in any case, but there nevertheless, among the primitive early vertebrates, is where genetic DNA analysis insists it belongs.
The Bottom Line: all these thousands of fish species can be genetically traced back, and back – back through the twigs and branches and limbs of countless thousands of generations over the millennia of long evolution, a many-braided stream – to a single fish-like creature, the ancestor common to them all.
The word “amphibian” means “double life” – i.e., in water and on land. All modern amphibians are thought to have evolved out of a common ancestor, and are today grouped in three classes: 1) frogs and toads, 2) newts, salamanders and mudpuppies, and 3) something called caecilians which does not mean “from Sicily” (the name actually means “worm-like, without legs”).
Other than certain wormlike caecilians, most amphibians are indeed four-legged and thus tetrapods, their limbs being thought to have evolved from the pectoral and pelvic fins of fish. Most amphibians have thin, soft skin, but caecilians have scales like fish. All amphibians are cold blooded, their internal body temperature depending on external heat sources. They have gills in their larval stage which are replaced by lungs in the adults (though some use cutaneous (through-the-skin) respiration supplementally or exclusively. They all lay eggs, but some eggs are fertilized externally (like those unrequited fish) while others enjoy the more popular internal fertilization procedure. Amphibian eggs lack shells, and are typically found covered with a thick gel floating in water. The larva stage of hatchling amphibians may last from ten days to twenty years. No amphibians are true ocean inhabitants, so are most commonly seen in freshwater ponds, lakes and streams.
1) Frogs and toads. “Toads” are in fact “frogs,” though the two have enough minor differences to justify the distinction. Both frogs and toads have no tail after they pass the aquatic tadpole stage.
Frogs have long well-muscled hind legs with webbed feet adapted for swimming and jumping. With smooth (sometimes slimy) skin they tend to prefer moist environments. They have bulging eyes and lay their eggs in clusters. Their more than four hundred species include the bull frog, green frog, common frog, leopard frog, march frog, wood frog and pickerel frog. They are found on all continents but Antarctica.
Toads comprise more than three hundred species, also found worldwide except in polar regions, Polynesia, Madagascar and Australasia (though toads have been artificially introduced into the latter regions). Toads have stubby bodies with hind legs made for walking instead of jumping, and so are typically shorter than frogs. They have dry warty skin and mostly prefer dryer climates. Many lay their eggs in long chains and a few species bear their young live. Many have defensive poison glands behind their eyes and the skin of some species bears toxins virulently deadly to even brief contact by a human touch.
2) Newts, salamanders and mudpuppies.
These lizardy-looking little amphibians all have stubby noses and long tails capping slender little bodies held barely off the ground by four short little legs. They all eat things like earthworms, snails and insects and their similarities exceed their differences. Belonging to three different families, they each develop in different stages. One-staged mudpuppies, for example, are fully aquatic and retain prominent gills throughout their lives. Two-staged salamanders are aquatic as tadpoles and terrestrial (mostly underground) as adults. Adult salamanders return to the water only for breeding, but this they do with gusto. Newts live through three stages: aquatic tadpole, terrestrial juvenile (called an “eft”) and – again – aquatic adult. Some more aquatically inclined newts manage to bypass the eft stage, and adult newts rarely venture onto land in any case.
Despite their scientific name “Gymnophiona,” caecilians are not normally found in gyms either. The word “caecilian” comes from the Latin “caecus” which means blind. In fact, some have bulging eyes, others possess small pseudo-eyes under their skin, and only certain ones actually lack any eyes at all. Adults range in size from three inches to nearly five feet. The 3,900 species of these legless serpent-looking tropical amphibians mostly live hidden in the ground (though some also live in water), thus are the least familiar to people who like to observe amphibians. With an appearance distinctly large-wormish or snakelike, it can be difficult to distinguish heads from tails. Of no particular interest to anyone but scientists specializing in caecilians, their skin is shiny and ringed with folds called “annuli.” Caecilians are predators possessing short sensory tentacles (located between their nostrils and where their eyes would be if they have eyes) which they use to probe their environment in search of prey. They normally eat soil-dwelling (real) worms and termites, but also will readily grab small frogs, lizards and snakes which they gulp down whole. They move like worms, come in many colors, and some have poison glands.
The Bottom Line: all these thousands of amphibian species can be genetically traced back, and back – back through the twigs and branches and limbs of countless thousands of generations over the millennia of long evolution, a many-braided stream – to a single amphibian-like creature, the ancestor common to them all.
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…to be continued in one week…
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