A living cell is a structure of immense complexity, comprised of a small bag of salty water filled with various polymer chains made from organic acids. Different polymers are clumped into different locations within the cell, to provide structural strength and to function as specialized production sites for energy generation, internal and exterior communication, waste management, infrastructure maintenance and recycling, and the construction of clones by cell division. Cells are, in fact, self-sufficient communities that harvest their environment for energy and construction materials, like small prehistoric communities of people, or like termite mounds. Different types of single cells have different sizes, ranging from a typical size of 0.01 mm for bacteria, up to over three pounds for an ostrich egg.
How such specialized machinery as proteins formed from the primordial soup, much less collected together form stable and self-sufficient communities is a major puzzle. And yet this happened relatively early in the 5 billion year history of the earth- within about 1 billion years after the crust first formed, according to the dating of stromatolite formations in shallow ocean waters. Varieties of cells have since proliferated to fill every available niche in the ecosystem, wherever there is any type of energy source and organic material on which to make a living, from deep ocean hydrothermal vents to Antarctic snow fields to upper atmospheric dust particles. The journey from unicellular life to multicellular structures took longer than did the first cell- about 2 billion more years- wherein different types of cells got together in cooperative efforts to communicate, metabolize and reproduce, forming meta-communities of the individual cells. A pattern was developing here.
It was only in the past 10% of earth’s history- about 0.5 billion years ago- that multicellular life really took off in the Cambrian Explosion… or at least generated sufficiently hard structures to form fossils that we can find. This breakthrough seems to have been triggered by the gradual transformation of our atmosphere- by cells that use chlorophyll-based metabolism- to provide an oxygen-rich environment for the oxygen breathing cell structures that are the basis of animal life. A new layer of community was added; plants using sunlight to turn CO2 into organic polymers + oxygen, and animal species using the plant material + oxygen to generate their own new cells + CO2… a perfect marriage.
The old, anaerobic cells never went away, but had to find environments away from the poisonous (to them) oxygen, in deep sea vents, underground compost pits, or inside the guts of animals where they partnered up to help with food digestion. A modern human is a vast community of different types of cells, most of which have been adopted from outside and bear someone else’s DNA; about 200 different kinds of DNA, with the total number of such visitor cells far outnumbering the number of human cells that are generated after the union of sperm and ovum. Since some types of bacteria are inimical to human health, it can be tricky business to choose antibiotics that selectively remove only the bad guys. Also, it is not uncommon for the human immune system to turn against some internal bacteria that mean no harm, and generate auto-immune responses that benefit nobody.
While the evolved interdependence of cells, bacteria, viruses, plants, animals, and humans within earth’s ecosphere is a marvel of efficiency and complexity, there are some cycles of dependency that are just bizarre. Herewith, from the site everything2.com, about the life cycle of a liver fluke that affects cattle:
The lancet fluke's lifestyle is a migration through the fluids of three hosts; a cow, a snail and an ant. Hungry snails eat the dung of the infected cows and swallow inadvertently the sequestered fluke eggs. Once hatching in the snail's intestine, they burrow through the gut wall and into a digestive gland. Within this gland, flukes reproduce a second generation - spewed back into the world by the tormented snail as balls of slime, each sticky drop, a seething mass of flukes.
It is in the third host, the ant host, where we observe mind control. Foraging ants come across the sparkling orbs of snot, sensing the source of moisture that they are, they quench their thirst on the slick beverage. Entering the third hypersea reservoir, the flukes undulate through the ant's fluids, most form cysts in the abdomen, but some home in on the nerve clusters that control the mandibles in the ants head.
The temperature drops into the coolness of evening, and the infected ant feels compelled to leave its brethren, forsaking them to climb a grass stalk spire to its apex - the pastures emergent canopy. Preparing to make itself a sacrifice, it anchors itself to the flimsy blade, attached firmly by its mandibles.
It waits motionless throughout the night to be devoured by the primary host, the cow. Herds like ruminating clouds pass over the ant, blotting out the stars, the hoof falls reminisant of distant thunderclaps. If the ant survives until morning the flukes relinquish their control allowing the ant to scurry back to join its fellow workers in the gloom and away from the solar furnace which would be death to both the host and the backseat driving parasite. By day the ant is a regular Joe indistinguishable from any other ant, but when night falls, again it makes its ascent into 'munch range' over and over until eventually consumed, drowned in cud, bursting open as a swarm of flukes within the cows stomach. The flukes complete the cycle by penetrating the bovines liver, becoming adult egg producers.
The liver fluke may be a champion in adaptability, but for real imagination in lifestyle, few can compete with the slime mold Dictyosteliomycota, the social amoeba. They spend their lives as individuals, grazing on bacteria on the forest floor. If food becomes scarce, however, the signal goes out and all individuals start crawling to a central spot. There they congregate to become a multicellular, slug-like creature that crawls over the floor to a new, more hospitable site. At the new site, the slug plants itself upon the ground and morphs into a plant-like structure, forming a fruiting body atop a stalk. The fruiting body casts out spores onto the wind, which settle on the floor to become a new colony of amoebas.
This blurry line between individual and communal behavior is echoed in beehives and anthills, where there is a case to be made that the significant organism is the community itself, with individuals being as expendable and replaceable as individual cells are within the human body. A blurry line also exists between life and non-life in the community of viruses. The tobacco mosaic virus can spend millenia in the form of a crystal, composed of aligned threads of RNA polymers. The crystal has a well-defined structure, which can be used to diffract X-rays like any other crystal. It seems the essence of crystalline solid matter, like table salt. But under appropriate conditions, the crystal disassembles itself to become an army of RNA virus threads that burrow into the cells of tobacco leaves, borrowing the cellular machinery of the leaf to generate more copies of itself. Such viruses cannot reproduce on their own, and so are considered non-living. This seems like a fairly arbitrary distinction; the concept of “alive” seems to be represented along a broad continuum, as perhaps does the concept of “consciousness”. But that’s a story for another time.
The remarkable feature of the evolution from organic acids to polymer chains to cells… all up the line to societies and national democracies is that- at every level of organization- there is a communal effort at work between individual components to form assemblies that communicate, compete, and cooperate with other assemblies. Complexity builds upon complexity. At each level of assembly, there enters the concept of another individual entity, e.g. an organism. a person, a tribe, or a nation. It is difficult not to see direction and purpose in this.
No comments:
Post a Comment