What People Say About Life
For thousands of years people have known how living things differ from lifeless things. For one thing, all the edible stuff is from Life. Of course, people had names for the different organisms, whether edible, inedible, poisonous, or otherwise dangerous. Some of these names grouped different but similar organisms into categories: bird, oak tree, grass, turtle, lizard, frog, clam. That is, people have long engaged in classification of organisms, with perhaps more refinement for edible and poisonous ones. Today we use the system invented by the Swedish naturalist Carl von Linnaeus (1707-1778), and published in his Systema Naturae, in 1735. He defined species and introduced the convention whereby each species receives a genus and species name (as in Mytilus edulis, the edible mussel). He also grouped genera into higher categories. His scheme has been adjusted by later taxonomists to yield the following sequence:
Carl von Linnaeus, the Swedish botanist who developed the system still in use for classifying living things.
Few people have a problem recognizing the kingdom level. We readily distinguish animals from plants and fungi (we know a dog from a tree from a mushroom, and we know they are all alive). At the phylum level, we find mollusks, arthropods and chordates (for example, snails and clams, crabs and flies, fishes and people). The class level contains familiar common names, such as coral, sea star, snail, spider, bird and shark. Order and family levels divide these large categories till we get to genus, and finally species. The most familiar example of Linnean classification is our own systematic position, as humans: Animals, chordates (subgroup vertebrates), mammals, primates, hominids, Homo, H. sapiens. (Linnaeus' label for humans as sapiens , the "wise" or "knowledgeable", suggests that Linnaeus had a high opinion of the ability to classify things, or else that he was a very generous person, or perhaps both.)
Linnaeus basically grouped the organisms according to similarity, as we do today, mostly. (Similarity may be measured by the amount of information it takes to describe differences.) Linnaeus had no scientific explanation why such groupings as he defined should exist (as he believed that species are permanent). The great French naturalist Jean Baptiste Pierre Chevalier de Lamarck (1744-1829), Professor of Zoology of Insects, Worms and Microscopic Animals at the Museum in Paris, proposed the correct answer. He insisted, in his book Philosophie Zoologique (published in 1809) that all organisms arose by evolution and are in a phylogenetic continuum. The more similar the organisms, the more closely related they are, by common ancestry. (This achievement of Lamarck's is usually credited to Darwin, in the popular literature and by many textbook writers. Darwin himself knew better. He wrote as follows: "In these works [Lamarck] upholds the doctrine that all species, including man, are descended from other species." 2nd page, 6th ed. of "The Origin of Species".)
One of the 10 surviving microscopes made by van Leeuwenhoek. (Courtesy: UC Berkeley)
Traditional natural science before Linnaeus recognized lifeless matter - minerals and rocks, water, air - and living organisms - animals and vegetables. Organisms grow and reproduce, animals can change their location. The world of microbes, of course, was not commonly known until the Dutch microscopist Antoni van Leeuwenhoek, 1632-1723, wrote about his observations. The world of organisms differs fundamentally from that of minerals and rocks in a number of ways, ways that define what we mean by "Life". Organisms consume matter and grow, and they reproduce. Invariably, they are part of a living support system, and they resemble (but are not identical to) other forms within that system.
You may wish to decide whether the attributes listed are sufficient for defining how an organism differs from non-living matter. Eating and replicating are nearly a full definition of life, but consider a fire, which eats (burns) fuel and can reproduce, by flying ambers for example. No wonder that fire was endowed with spiritual powers by primitive religion (just like trees and other living things). One thing that fire does not do is evolve to make different species of fire.
Perhaps, then, we can agree on this definition: To be alive, a recognizable adaptable system has to have a means for limitless replication (given the right conditions) with room for error and improvement. Note that we left off the point about eating (metabolism), which concerns the maintenance of the system. For all we care, a living system can be dormant until it replicates. So this is a "replicationist" view, as opposed to the "metabolist" view, where growth is the center of attention and replication need not be accurate at all.
Possibly, in the early stages of life's history, there were life forms specializing in growing, and there were others specializing in replicating. Perhaps, when they joined forces, Life was on its way to success.
Here is what scientists active in the field have to say:
"Life is an entity that can replicate itself from far simpler parts, and which is subject to evolution." - Carl Woese
"Life is a self-bounded system where the boundary is made by the material in the system. It is a process involving the production and maintenance of identity." - Lynn Margulis
"Life begins as a loose scum of replicating molecules without a boundary." - Jeff Bada
"Life can be recognized by what it does; living organisms create hallmark molecules and create chemical disequilibrium." - Ken Nealson
Note that viruses have no metabolism; they are minute replicating molecules that can harm the host cells they inhabit. Prions, the agents of BSE or "mad cow disease", are even smaller than viruses and seem yet stranger. Neither viruses or prions, it seems, can exist without living organisms, but they do not themselves fulfill the common definitions for life.