It's time, as the old song says, to accentuate the positive. My last couple of posts took a pretty jaundiced view of the living world, focusing on some of its more ghastly creatures. They're out there, but I don't want to dwell on them too much. Nature has produced plenty of wonder and beauty, too. What's most wondrous to me is the diversity of life--the amazing variety of ways to be a living organism. There are nine-pound crabs that climb trees but can't swim, and spiders that live their whole lives under water, wearing a web-bubble full of air. Bacteria flourish under Antarctic ice, and algae turn snowfields pink high in the mountains. In 1995, scientists found a creature that lives only on the mouth bristles of lobsters. I'm convinced you could study life on Earth for a thousand years and and never run out of surprises.
Life has been evolving for at least 3.5 billion years, creating millions of different species, each with its own unique--and often completely astounding--way of getting by in the world. The best estimates put the number of species on Earth today at around 8.7 million, and the ones living today are just a small percentage of all the species that have ever lived. The earth has seen entire dynasties of living organisms rise, diversify for tens of millions of years, and finally go extinct.
Our species, then, is a single living branch on an ancient tree; one of mllions of other branches. Before I talk about some of the weirder branches, it's worth mentioning a couple of things about the tree's major limbs, because our understanding of those has changed a lot in the last 50 years or so. There's still a lot of misunderstanding out there--lots of people still think you can classify every living thing as a plant, an animal, or a microbe. It's not that simple.
Early taxonomists like Linnaeus classified the natural world into three kingdoms: animal, vegetable, and mineral. Linnaeus classified living things using a hierarchy of categories (kingdom, phylum, class, etc.). One kingdom includes multiple phyla, which themselves include multiple classes, and so on. This system can be represented as a branching tree, but Linnaeus lived before Darwin, so he thought of it more as an organizational chart than a genealogical tree. In the mid-1800's, though, scientists started thinking of living things changing and diversifying over time, and Darwin explained the basic mechanism by which that happened. The tree of life turned out to be a true family tree.
The more biologists learned about the tree of life, the more complex it got. Microscopic organisms were given their own kingdom, Protista, as in this 1866 diagram by Ernst Haeckel. Then people realized that life has even broader divisions than kingdoms. Two of the deepest branches in its tree are between
prokaryotes--simple cells with no nucleus, like bacteria--and
eukaryotes, whose cells are more complex, and nucleated. Then prokaryotes themselves were found to contain two distinct groups: bacteria, and another kind of microscopic organism called archaea. Fungi turned out not to be a kind of plant, but an entirely separate kingdom, more closely related to animals.
Today, most biologists divide the tree of life into three domains: Bacteria, Archaea, and Eukaryotes, as in the tree below. Each of these domains has deep divisions that correpond to kingdoms. Animals, plants, and fungi are still considered kingdoms within the eukaryotes, but so are other groups of eukaryotes. Single-celled eukaryotes like amoebas, giardia, and euglena are informally referred to as protists, but that's just a term of convenience, not a real biological group. Some non-animal/plant/fungi eukaryotes, like kelp, are multicellular and big enough to form underwater forests.
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| From Wikipedia |
Now let's get into the fun stuff--the amazing diversity of lifestyles in nature. As animals, we get our energy and basic building blocks (organic molecules) by eating other organisms. Plants, of course, make their living in a totally different way. They get energy from the sun, and then use it to assemble their own organic molecules from scratch. Most people figure that since plants grow out of the dirt, they must be made of dirt. But they aren't. Soil just provides minerals and other nutrients. If you weigh the soil in a pot, then grow a five pound plant in it, and then weigh the soil again, you won't find much difference before and after. Plants literally build themselves out of air and water, using sunlight. Carbon dioxide in the air provides the carbon that forms the backbone of organic molecules. This trick, of course, is called photosynthesis, and it makes life possible for most living things--even the ones, like us, who can't do it. But plants didn't invent photosynthesis. Bacteria learned it long before plants existed, and so did many single-celled protists (some of which hedge their bets and eat other organisms too).
Some bacteria and archaea get by in more exotic ways. Many can build their own organic molecules without light, using energy from chemicals like sulfur, iron, and even ammonia. Some of these weird organisms support ecosystems that grow in total darkness, around boiling volcanic vents at the bottom of the sea. There they form symbiotic relationships with seven foot tubeworms, who shelter them inside their bodies. Not how I would want to live, but it works for them.
Unexpected symbioses like that are common. Lichens are a symbiotic amalgam of algae and fungus. Termites and cows rely on bacteria in their guts to digest cellulose. Figs need tiny wasps inside their fruits in order to reproduce, which is why some vegans and squeamish biology majors won't eat figs. The bobtail squid of Hawaii harbors symbiotic bacteria that allow it to glow in the dark. Kudzu, soybeans, and alders develop nodules on their roots to house bacteria that help them use nitrogren.
In plants and protists, photosynthesis itself is based on an ancient symbiosis. Chloroplasts, the photosynthetic cellular organs inside their cells, originated as bacteria that took up residence inside other organisms. Sometimes this process happened twice. For example, red agae is a kind of seaweed that isn't related to plants. Once upon a time, a single-celled ancestor of red algae engulfed photosynthetic bacteria and gained the ability to photosynthesize. Later, other eukaryotes called chromealveolates engulfed the red algae cells, creating a double-layer of internal symbiosis. They went on to form a major lineage of eukaryotes, which include several kinds of algae, as well as kelp, diatoms, and the parasites that cause malaria. It's a family with diverse interests. The red algae, meanwhile, formed several multicellular lineages, some of which get hardened with calcium and build reefs alongside corals (which are colonial animals).
It's a strange, unexpected world out there. Think of the amazing variety just among the plants. There are plants in the desert called living stones that avoid herbivores by looking like rocks. Here in Louisiana I see resurrection ferns along the limbs of live oak trees. They get their name because they look stone dead when it's dry, but then they have a green resurrection as soon as it rains. The titan arum of Sumatra produces a flower that can be ten feet tall. It's pollinated by flesh flies and carrion beatles, which is why it smells like something crawled into it and died.
Plants have been co-evolving with insects for millions of years, forming some truly astounding relationships. Bee orchids get pollinated by tricking bees into trying to have sex with their flowers. Spider orchids trick wasps into stinging them. Darwin's orchid, like many flowering plants, bribes insects with nectar. But it keeps it at the bottom of a tube that can be over ten inches long. When Darwin saw it, he predicted that a moth would be found with a 10-inch proboscis, long enough to reach the nectar. The moth was discovered years later, after Darwin was dead.
The organisms that most capture the human imagination are our fellow animals. They're a stunningly diverse bunch, from lowly sponges and hideous tapeworms to lions, tigers, and bears. Animals have some strange lifestyles and behaviors themselves. There are male jumping spiders that flare their backsides like peacocks and wave their arms in the air to woo females. Scallops, which look like flattened clams, can swim away from predators by flapping the two halves of their shell together. Water beetles breathe through their rear ends (and some turtles can do the same thing). The boxer crab, also known as the pom-pom crab, is so-named because it holds a tiny, stinging sea anemone in each of its pincers. When threatened, it gives its opponent an anemone sandwich. Sometimes nature is dowright hilarious.
Our own group of animals, the vertebrates, is also full of surprises. Consider the world of fish. The first surprising thing about them is that they aren't a coherent biological category. Cartilaginous fish like sharks and rays are very different from bony fish like catfish or bass, and those, in turn, are different from lobe-finned fish like coelacanths (famous "living fossils") or lungfish. Lobe-finned fish were the ancestors of four-legged vertebrates, which means a lungfish is more closely related to us than it is to a shark.
Fish have been around long enough to evolve into some truly weird forms. The gulper eel is basically a giant set of jaws with a tail, and it can swallow things much larger than itself. The tip of its tail glows, too, in case it wasn't freaky enough already. The eyelight fish, one of several kinds of flashlight fish, has bioluminescent lights under each eye. The barreleye has eyes on barrel-shaped stalks, enclosed inside a transparent dome on its head. The deep sea anglerfish is famous for the glowing lure that hangs from its forehead, and for being butt ugly. But the weirdest thing about it is its sex life. Male anglerfish are tiny, and when they find a female they attach themselves to her permanently, and then sort of dissolve on the inside, basically becoming a bag of gonads. Some females have five or six of these dangling off of them. Who says romance is dead?
Then there's the flying fish, famous for leaping out of the water and gliding for hundreds of feet on wing-like fins. This is so well-known it's easy to forget how odd it is--this is a fish we're talking about here. But gliding isn't unusual among vertebrates. It's evolved several times. Wallace's flying frog flares out its fingers and toes and glides on the membranes in between. Flying lizards flare out their ribcages as wings. Flying snakes flatten their bodies, launch themselves out of trees, and glide by slithering through the air. Among the mammals are the flying lemurs and flying squirrels. They both glide with flaps of skin between their front and back legs, but they aren't closely related, which means their abilities evolved separately. Gliding must be a handy trick.
Gliding is just one of many weird, wonderful traits among animals. Possibly the most amazing of all, for my money, is this: baleen whales--the largest animals that ever lived--have an expandable mouth, like a pelican. They can hold more than their weight in water in their mouth...and some of them weigh nearly 200 tons. They take in all that water, strain krill out of it, and spit it out again. These behemoths are out there doing that right now, in oceans around the world.
That kind of fact boggles my mind. But it's just one of many such facts about animals. And animals are just a part of a huge, diverse tree of life, and even life is just one aspect of the natural world. It's easy to get carried away talking about all the wonders of nature, and that's just what I've done in this long, rambling post. I can't help it. It's an astonishing world out there.
