(continued) Chapter 3. Long Evolution: Life Emerging
There are two ways of arriving at the truth. I decided to follow them both.
(Georges Lemaitre, Belgian astronomer, physicist, Catholic priest, who first proposed that the universe expanded from a hot point of origin which we now call the big bang)
One puzzling question is why there should be only two solutions.
(Sir Arthur Eddington, British astronomer, physicist, mathematician, who confirmed Einstein’s theory of general relativity to be correct)
The entire cosmos is suffused with sentience. We are surrounded and immersed in consciousness; it is in the air we breathe, the soil we tread on, the bacteria that colonize our intestines, and the brain that enables us to think.
(Christof Kock, in Consciousness: Confessions of a Romantic Reductionist)
There’s a couple of different ways to look at the eukaryotes. One is the fairly boring way in which we looked at bacteria and archaea, which are what they are and cannot much rise above boring even at their Sunday best. Here again, as with the bacteria and archaea, the great majority of individuals consist most boringly of one single cell. You’d probably say “They all look alike” even if they don’t think so. If we were to let things rest at this level of boredom, the eukaryotes (which, I remind you, includes us) would look something like the following.
The major groups of eukaryotes (boring version)
From the first/oldest (low consciousness) to latest/most complex (high consciousness):
- Diplomonads (one-celled creatures having a propeller or other wavy tail things they wiggle to move themselves toward food. Most un-diplomatically, most diplomonads are parasitic predators, the wealthier of which have a “feeding groove” on one side. This groove is genetically related to a feature that evolved separately in other creatures and is called a “mouth.”)
- Microsporidia (a parasitic fungi group that practices unsatisfying sex by releasing spores into the open air. Love’s labor lost, they nevertheless reproduce with considerable success.)
- Trichomonads (inhabiting animals’ digestive tracts, some are symbiotic, others parasitic. Mobile and one-celled, they reproduce simply by dividing in two.)
- Flagellates (at some point in their life cycle, these small creatures have flagella for locomotion and sensation)
- Entamoebae (parasitic in animal bodies (mainly humans and other primates), these disgusting little entities cause sleeping sickness)
- Slime molds (several kinds of unrelated eukaryotes living freely as single cells, though many also form large multi-cell structures. Found mostly in forests, slime molds were for quite a while wrongly classified as fungi. They in fact like to eat fungi as well as bacteria, decaying vegetation and other slime molds.
- Ciliates (possess hair-like cilia that look just like flagella but (presumably significant to them) their cilia undulate differently than flagella)
- Plants (multi-celled eukaryotes of the order plantae. Includes trees, bushes, ferns, mosses, hornworts, liverworts, and green algae. Plants do not include red or brown algae, fungi, archaea, bacteria or animals.)
- Fungi (yeasts, molds and mushrooms, among other things)
- Animals (everything else that’s not bacteria or the archaea named earlier in this chapter)
Eukaryotes re-sorted (possibly more interesting version)
That’s one way to look at eukaryotes. Boring. But we don’t have to let them rest at this level – we can spruce up the interest level and forthwith hereby do so. In this rendering (experts can differ, you know), the ten genetic (DNA-based) groups are collapsed into a mere five which seem perfectly adequate and less difficult to remember. They are:
Though having risen to the level of eukaryotes, the vast majority of these individuals are still little bitty things. Even if they have more than one cell, most of our fellow eukaryotes make us plants and animals look pretty complex. A source of pride, perhaps.
The five types of eukaryotes (possibly more interesting version)
1. Chromista. Chromista means “colored” though a few (e.g., mildews) are colorless. Mostly photosynthetic like plants but unrelated to plants, chromista include diatoms, kelps and haptophytes. Their planetary distribution and range in size are colossal.
Diatoms, microscopic chromists with delicate silica skeletons only forty millionths of a meter long, are a primary food source for many fresh-water and ocean-going fish and other animals. Giant kelps, by contrast, grow up to fifty meters long, line continental coastlines and are commercially important for fish and shellfish feeding and reproduction. Haptophyte chromists are mostly single-celled phyto-plankton that provide sustenance for sea dwellers such as whales, jellyfish, snails and shrimp, and their position at the base of the food chain is exceedingly important in aquatic ecosystems. Chromists contribute to many commercial products (e.g., paper production, toothpaste, ice cream). The bodies of ancient chromists are still around as deposits of limestone, silica and calcium carbonate.
2. Fungi. Fungi too have important ecological and economic roles. By breaking down dead organic material they recycle nutrients back into ecosystems. Their symbiotic presence on plant roots converts soil minerals to nutrients that enable the feeding and growth of most vascular plants. Fungi provide edible mushrooms and numerous drugs (e.g., penicillin), and commercially valuable fungi such as yeasts provide the bubbles in champagne, beer and bread. Some fungi cause plant crop diseases like corn smut, and animal diseases such as ringworm and athlete’s foot – difficult to treat because fungi are genetically closer to animals than they are to bacteria and archaea.
3. Protists (also “protoctists” but not proctologists). “Kingdom Protista,” a fairly recent new subdivision of eukaryotes, is defined more by the absence of characteristics (such as lack of extensive cell differentiation) than by the unique attributes that normally define life categories. Protists are not “monophyletic,” which means the group contains some organisms more closely related to members of other kingdoms than to other protists. As a category of classification, protists remain in flux (experts differing again). Recent studies of their DNA and structure show that protists are more diverse than previously thought and probably should be subdivided into several kingdoms. Experts will decide, somehow.
4. Plants. With more than a quarter-million species, the plant kingdom is second in size among eukaryotes only to the arthropods (insects, spiders-arachnids, centipedes-millipedes and crustaceans). Plants have been around almost half a billion years, though familiar modern seed-producing forms didn’t appear until about 360 million years ago. Plants take on their green color from use of chlorophyll to capture light energy for making their food – sugar, starch, and other carbohydrates. Vital shapers of the environment and builders of soil nutrition for their own seeds, plants are found profusely everywhere on earth except arctic wastelands, extreme deserts, and the deep ocean. Human life could not exist without plants providing the fruits, vegetables and grains we rely on, and feeding the animals which likewise eat plants and supply humans with food and power. Different expert sources, as you can imagine, classify the plant kingdom in different ways. I find the following four-part subdivision useful and easy to remember.
Mosses. As land plants lacking seeds or flowers, the large family of mosses (and their close relatives, the ever popular liverworts and hornworts) reproduce by spores that are released in two alternating generations. In the first “gametophyte” generation, the larger and more easily seen surface plant produces mature germ cells (gametes) that unite with another of the opposite sex in sexual reproduction to form a zygote. In the second phase of its dual cycle, the subsurface “sporophyte” generation then produces the spores. Rarely seen, this phase is a small plant growing on or just under the soil, and revealed by a tiny bulb on a thin vertical stalk. Lacking internal vessels, mosses grow only as small individuals, normally in moist environments.
Ferns, horsetails and a few related species (together called pteridophytes) make up the second large group of land plants. These do have vascular systems which carry water and nutrients from the roots up to the leaves (often called fronds), then carry sugars and other metabolic products back down to the roots. Like the mosses, ferns lack flowers and reproduce by means of spores. The source can be seen on the underside of the fronds as little dots which produce thousands of spores. Also like mosses, ferns and horsetails are bi-generational. In a gametophyte generation, a spore grows into a very small plant that produces gametes (eggs and sperm) which unite to produce the mature plant. The mature plant then produces new spores in the sporophyte generation.
Gymnosperms (“naked seeds”) make up the third great family of land plants. The four gymnosperm groups include cycads (palm-like plants), the ancient ginkgo tree, conifers (reproduce by seeds contained within a cone) and the genetophytes (an odd assortment of everything else that’s a gymnosperm). Conifers represent a significant evolutionary development since, unlike spores, the seeds are multicellular and contain nutrition within a protective casing for the new infant plant. Gymnosperms include the most massive, tallest and longest-living plants on earth – redwoods, Douglas firs, sequoyahs – and these too are bi-generational. In phase one conifers produce pollen grains, which are immature male gametophytes. The pollen reaches female cones by transport on the blowing wind, a most inefficient process, so conifers compensate by producing large amounts of pollen to ensure fertilization of female cones. A female thus fertilized with pollen then produces seeds for the phase two – sporophyte – generation.
Angiosperms – “flowering plants” – are the fourth great plant group. To date science has identified and named roughly 400,000 separate plant species, of which fully three-fourths are flowering plants – from moss rose and zinnias to deciduous trees and rainforests. This large group of land plants, perhaps the most familiar to most people, represents the high point of plant evolution. Its distinctive feature is the flower, a cluster of quite specialized leaves which figure prominently in the plant’s reproduction.
The flowers of some plants are conspicuous (Magnolia blossom), some are barely discernable. Oaks, ivy, and grasses all produce flowers, but we seldom notice because they aren’t showy. Regardless, they all manage to produce seed – the mighty oak’s acorn is a familiar example. The angiosperm flower’s scent and color attract many mobile species (insects, birds, animals) to touch the flower, unknowingly picking up grains of pollen which they carry afar and thereby assist pollination. Some scientists, in Richard Dawkins’ footsteps, like to say this is “the flower’s genes controlling our genes to accomplish its desires.” I say this is a nutty and perverted gross misinterpretation of one of nature’s natural little processes, inspired by a closed mindset intent on denying every last possibility that Nature’s many little mysteries might *gasp* imply the existence of God, or a god, or gods, whatever. A waste of mental energy. Nuts.
The angiosperm’s clever little ways of achieving pollination clearly are less random and more efficient than is the case for the gymnosperm/conifers. The angiosperm seed develops inside an “ovary,” which we animalia think of as edible “fruit” and thus delight in eating We humans eat them, monkeys eat them, bears love them. The natural attraction of animals to tasty sweet fruits enables seed dispersal as we eat the seeds and pass them generally unharmed through our intestinal tracts. (It also indicates humans aren’t the only species that possesses and adores the taste of “sweet.”) Because of our great mobility, animals, insects and birds often deposit the seeds, along with a bit of fertilizer, far way from the original plant, thereby achieving plant “emigration” to new habitats. Very effective, implying nothing religious one way or the other.
Flowering plants also are economically important. Arrangements in vases aside, we acquire nearly all our food, ultimately, from flowering plants. Fruits, vegetables, beans, nuts, grains, spices, herbs tea, coffee, chocolate, wine, beer, and colas come from flowering plants. Natural fabrics such as cotton, linen, rope and burlap are made from the fibers of flowering plants, and many commercial dyes are extracted from them. Flowering plants provide many of our drugs such as aspirin and digitalis, and controlled drugs such as tobacco, marijuana, opium and cocaine.
There is much more to be learned on the subject and some people spend whole careers focused on flowering plants alone. Before moving on, we have opportunity to learn a bit more about those conifers and flowering plants, so dear to our lives, called “trees.” Writing in The Roots of a Naturalist (Smithsonian Magazine, March 2012), Jane Goodall notes that trees first appeared long millions of years ago when fern-like plants definable as trees began spreading over land surfaces, sending down roots which broke up the hard surface and eventually enabled the first forests to form. She says:
“The Archaeopteris, which flourished…385 to 359 million years ago, is the most likely candidate so far for the ancestor of modern trees. It was a woody tree with a branched trunk, but it reproduced by spores, like a fern. It could reach more than 30 feet in height, and trunks have been found with diameters of up to three feet. It seems to have spread rather fast, occupying areas around the globe wherever there were wet soils, and soon became the dominant tree in the spreading early forests, continuing to remove CO2 from the atmosphere.”
Highly adaptive trees indeed were increasingly important agents for removing carbon dioxide from the early atmosphere as well as cooling it to temperatures more amenable to the spread of oxygen-loving animals. Once established, early forests enabled both plant and animal species to emerge in more and more habitats. Today an estimated 100,000 tree species inhabit the planet.
The Bottom Line: all these thousands of plant 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 plant-like life form, the ancestor common to them all.
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This brings us to the most interesting of all – the animals. Which of course includes us.
5. Metazoa (ANIMALS)
Like the other eukaryotes above, animalia come in several broad designs. Here’s most of them, briefly, with their enchanting scientific names only a scientist could love:
Ecdysozoa: Largest group in the animal kingdom, containing both arthropods (insects, crustaceans, spiders) and nematodes (microscopic worms. The most numerous muilti-celled animals on earth, many are revoltingly parasitic.)
Lophotrochozoa: Bilaterally symmetrical with a left and a right side to their bodies, this major group includes annelids (ragworms, earthworms, leeches), brachiopods (marine animals with hard shells) and mollusks (snails, clams, mussels, squid, octopuses – the first three of which also have hard shells.)
Deuterostomia: Scientists continue to debate the genetic relationships among this group’s highly diverse animals. Its most widely recognized subdivisions are echinoderms (sea stars, sea urchins, sand dollars, sea cucumbers, sea lilies and others with five-point symmetry) and their close relatives, the vertebrates (which include us).
Notice: we humans are more closely related to sea cucumbers and sand dollars, creatures of obviously low intelligence that just lay around, than we are to clams and octopuses. While clams don’t have much to say, some octopuses clearly display quite high levels of intelligence. This means that octopuses – non-vertebrates, outside the genetic chain that led to mammals, without reference whatsoever to humans – evolved a significant level of intelligence all on their own. As Sam Clemens would ask, Now what do you say to that?
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…to be continued in one week…
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