After more than 200 years of classifying organisms, you'd think scientists would have agreed on a grand plan for life. Maybe not the details, like the number of plant species or anything, but at least an overall vision.
Think again. Once upon a time, scientists thought life could be slotted into two kingdoms (over-arching groups): plants and animals. Around 1860, when scientists realized that microorganisms didn't fit that scheme, they created a third kingdom, protista to include single-celled organisms.
But that plan faced difficulties, too, and today you can choose from several more sophisticated schemes for classifying life.
Biologist Lynn Margulis of the University of Massachusetts and her colleagues prefer to split life into these five kingdoms:
MONERA prokaryotes (cells without nuclei); includes
all bacteria, and the vast majority of life forms and more biodiversity
than the rest of life put together.
PROTOCTISTA eukaryotic microbes (cells with nuclei), products of bacterial symbioses, and their descendants; includes algae, ciliates, and other microbes that don't fit other categories
FUNGI spore-forming molds, yeasts, and mushrooms
PLANTAE maternally retained embryo-formers
ANIMALIA animals; products of an egg fertilized by a swimming, smaller cell (a sperm)
(Classification adapted from "The Illustrated Five Kingdoms" -- see our bibliography.)
A
molecular vision
That was the challenge facing University of Illinois biologist Carl Woese back in the 1970s. Woese took a new tack, employing the emerging tools of molecular biology to look at the sequence of RNA in the ribosome (defined).
Ribosomal RNA is handy because it changes slowly and randomly over the millennia. Hence the greater the variation between the genetic sequences of two organisms, the greater the evolutionary distance will tend to be between them, and the more remote in the past was the common ancestor they shared.
Woese says classifying organisms by genetic sequences rather than appearance "Gives us an evolutionarily panoramic view of the microbial world, and thereby helps us understand an awful lot of microbiology, and about the underpinnings of life on this planet, which are microbial." Before sequence-based analysis, he says, "the whole microbial world was terra incognita because they could not enter our concepts."
Does this system place undue significance to genetic sequences at the expense of structure, the traditional basis for classification? Although "all classification systems are arbitrary," Woese counters, he says he agrees with Charles Darwin, who recognized that the goal of classification schemes is to explain how organisms are related: "Genealogy by itself does not give classification...[but] our classifications will come to be as far as they can be so made, genealogies."
And there's another advantage of using genetic sequences, at least when it comes to bacteria and Archaea: you don't have to even grow the bug to learn something about it: all you need is to extract DNA or RNA and analyze it. That's particularly handy for the many microorganisms that can't be grown in the lab because scientists don't know what conditions -- temperature, pH, nutrients, and symbiotic organisms -- for example, they require to grow.
Traditionally, taxonomy (defined) has classified organisms by appearance, on the (correct) assumption that similar structures indicate -- if not quite prove -- common ancestry. But what to do about the huge number of microbes, whose appearance, if known at all, could be misleading?
Using
genetic analysis
Even if you haven't heard "Archaea" until today, they made big news -- yes, real news in August.
Woese ended up with an entirely different view of the biosphere: as divided into three kingdoms, with the famliar plants and animals occupying to a small portion of one kingdom:
That may seem revolutionary (or humiliating) to creatures that have grown accustomed to thinking of themselves as the most important organisms in the universe, but Woese says "that's the way it is."
