Monday, March 1, 2010

Plants and Animals

by Richard Crews
Fifty years ago when I studied biology in college, all living things were divided into two broad groups or "Kingdoms": the plant Kingdom ("Plantae") and the animal Kingdom ("Animalia"). To be a "living thing" you had to demonstrate several key abilities such as procreation (the ability to make more little guys like yourself) and metabolism (the ability to harvest energy from the environment to run your inner machinery). The difference between "animals" and "plants" was also simple: if you could move yourself around, you were an animal; if not, you were a plant.

Within each of these two Kingdoms the creatures were divided into "phyla" (the plural of "phylum"); each of these into "classes"; these, into "orders"; then, "families"; "genera" (the plural of "genus"); and "species." So we biology students memorized "Kingdom, Phylum, Class, Order, Family, Genus" (all capitalized), and finally, "species" (not capitalized)." It was a neat and orderly system that scientists had been developing for a couple of hundred years; more than two million organisms had been closely studied and classified into this system.

Then starting about 50 years ago with the advent of better and better tools (such as the electron microscope) and more and more biologists making more and more careful and sophisticated studies, things got more and more complicated. First there was another Kingdom added (I remember with wistful nostalgia how having just two Kingdoms--plants and animals--had seemed so tidy). Then another, and another. FIVE Kingdoms! Clearly things were getting out of hand.

But there continued to be better and better tools (like DNA sequencing) and more and more biologists picking over things.

Nowadays Kingdoms are not even the highest level any more; there are three super-kingdoms or "domains" of living organisms. (It is even time to stop capitalizing the word "kingdom.") One of these domains has four kingdoms. Another has three. And one of the domains, the bacteria, hasn't even had its members divided up into kingdoms yet; instead the official word is "kingdoms are not yet available"; there are said to be five "divisions" among the bacteria but beyond that--well, hold the phone.

Today I heard a high-school biology teacher lament that "next year I'm going to have to change my 'Kingdoms of Life' chart again." (She meant her "Domains of Life" chart.)