Life

What is life?

Philosophers, scientists and other great thinkers have been asking that question for centuries. Today, we still don’t know, even though scientists have classified living things into as many as six kingdoms. GeoZoo focuses primarily on the kingdom Animalia, especially vertebrates, which belong to a subphylum (Vertebrata) of the phylum Chordata.

If you visit www.geozoo.org/Life/Chordata, your browser should look something like the image below, except that subphylum Vertebrata’s children won’t be visible until you click Vertebrata. Notice that the chordates’ parents (Animalia and Life) are listed at the top of the page. Below the title and subtitle are two tabs, labeled My Groups and My Children. Clicking My Children opens a list of all the members of the next lower taxonomic level that belong to a particular group. For example, Chordata’s children are the subphyla Vertebrata and Urochordata.

If we eliminate intermediate taxonomic divisions, like superorders and subphyla, then Chordata’s children would consist of the various vertebrate classes (birds, mammals, etc.), along with classes belonging to Urochordata.

My Groups might be thought of as the various clubs a particular animal or group belongs to. For example, the sea otter’s groups might include Marine Mammals, Tool Users and Predators. (By the way, the links in the image above work, including Animalia.)

Classifying Life

Actually, most people instinctively know what life is. It’s just hard writing a precise definition. Things also get more confusing when we peer through a microscope — or telescope. We know that animals and plants are living things, and soil and water aren’t. But what about algae, fungi, bacteria and viruses?

In fact, the great Swedish scientist Linnaeus (1707-1778) faced that same situation nearly three hundred years ago. Linnaeus set out to classify living things, placing them in two kingdoms, plants and animals. (He called them Animalia and Vegetabilia.)

Linnaeus divided each kingdom into classes, later grouped into phyla for animals and divisions for plants.

Micro Life

Linnaeus’ kingdoms are still generally recognized, except that Vegetabilia has been divided into two kingdoms, Plantae (true plants) and fungi (mushrooms, etc.). However, the scientific world was rocked by the discovery of microscopic creatures, about the time Linnaeus was born.

A Dutch scientist named Anton van Leeuwenhoek (1632-1723) is generally credited with bringing the microscope to the attention of biologists. He is also commonly known as “the Father of Microbiology.”

When single-celled organisms were first discovered, mobile forms were classified with the animal phylum Protozoa, while colored algae and bacteria were assigned to the plant division Thallophyta or Protophyta.

Evolution

In the 19th century, the scientific world was rocked yet again by a new concept called evolution. It further influenced ideas regarding the classification of life, with scientists focusing on lines of descent rather than sometimes superficial similarities.

By the late 19th century, just about every living thing other than animals and plants (which still included fungi) was placed in a kingdom called Protista. In 1937, Edouard Chatton divided all living things into two “empires” based on their cell structures. Eukaryota (whose cells include nuclei) includes animals, plants, fungi and protists. Living things without a nucleus were placed in Prokaryota.

Chatton’s vision became more popular in the 1960’s, as classification schemes continued to evolve. By then, it was popular to divide multi-cellular life forms into animals, plants, fungi and protists — collectively called eukaryotes. (However, some protists are single-celled.) Prokaryotes were commonly placed in the kingdom Monera. By 1977, the Monera had been divided into the Eubacteria and Archaebacteria for a total of six kingdoms in two domains.

In 1990, Carl Woese proposed that the Eubacteria, Archaebacteria, and Eukaryota represent three primary lines of evolutionary descent. Accordingly, he created three domains. Animals, plants, fungi and protists make up the domain Eukarya (the eukaryotes), with single-celled organisms divided into Bacteria and Archaea.

The animated image on the left illustrates seven stages in the evolution of the classification of living kingdoms. The image on the far left is GeoZoo’s interpretation. The more familiar life forms are grouped together in the kingdoms Animalia, Plantae and Fungi. Though they are also eukaryotes, protists are grouped with the single-celled life forms because they are less familiar. (The most familiar protists include algae and slime molds.)

A third group at the bottom includes other possible life forms. Scientists aren’t certain whether or not viruses merit classification as living things. The existence of life on other planets is another subject of scientific debate. In the meantime, some scientists and philosophers have suggested that Earth itself can be considered a living thing. The late philosopher Alan Watts suggested there may be many planets that produce living things the way trees produce fruit.

There are still more possibilities. Soil, which is composed of countless micro-organisms, has also been nominated as a living thing. And who knows what extinct life forms remain to be discovered? What about computer viruses and software programs and robots of the future? Is there also a spiritual realm?

But What is Life?

No single thing defines life. For example, one might think that anything that grows, responds to stimuli and reproduces is a living thing. But, by this definition, fire would be a living thing. Fire is even mobile, somewhat similar to animals.

A Conventional Definition

Not surprisingly, there is no universal agreement on the definition of life. However, scientists generally associate the following phenomena with life (taken from Wikipedia):

1. Organization - Living things are composed of one or more cells, which are the basic units of life.
2. Metabolism - Metabolism produces energy by converting nonliving material into cellular components (synthesis) and decomposing organic matter (catalysis). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life.
3. Growth - Growth results from a higher rate of synthesis than catalysis. A growing organism increases in size in all of its parts, rather than simply accumulating matter.
4. Adaptation - Adaptation is the accommodation of a living organism to its environment. It is fundamental to the process of evolution and is determined by the organism’s heredity as well as the composition of matabolized substances, and external factors present.
5. Response to stimuli - A response can take many forms, from the contraction of a unicellular organism when touched to complex reactions involving all the senses of higher animals. A response is often expressed by motion. Examples include the leaves of a plant turning toward the sun or an animal chasing its prey.
6. Reproduction - The division of one cell to form two new cells is reproduction. The term is usually applied to the production of a new individual (either asexually, from a single parent organism, or sexually, from at least two differing parent organisms), although, strictly speaking, it also describes the production of new cells in the process of growth.

A useful characteristic upon which to base a definition of life is that of descent with modification (or reproduction combined with evolution): the ability of a life form to produce offspring that are like its parent or parents, but with the possibility of some variation due to chance. Descent with modification is sufficient by itself to allow evolution, assuming that the variations in the offspring allow for differential survival. The study of this form of heritability is called genetics.

Howevr, this argument includes replicating elements which have been observed to evolve but are not generally accepted as being alive. Examples include viruses, plasmids, transposons, satellite DNA. Another exception might be the software code of certain forms of computer viruses and programs created through genetic programming.

Exceptions

So what about sterile castes (such as worker ants) and hybrids (e.g. mules), which can’t reproduce? The secret is to apply the definition of life to a species, not to individuals. In fact, even animals that don’t reproduce may contribute to the survival of their species.

The Origin of Life

We might find an answer to the question “What is life?” if we knew how it originated. Unfortunately, that’s little more than a cosmic guessing game at the moment.

Evolution: On Earth or Mars?

It is generally believed that life on Earth evolved here on Earth. Scientists often describe ancient Earth as a “primordial soup” of natural ingredients and phenomena that interacted with each other and eventually formed the basic building blocks of life.

Some people have speculated that Earth might have been colonized by life from another planet. The most spectacular evidence supporting this theory is ALH 84001, a meteorite discovered in Antarctica.

The meteorite was discovered in 1984, some 13,000 years after it struck Earth. The second most amazing thing about this space rock is its alleged origin: Mars.

Even more amazing was the announcement, in 1996, that the meteorite harbored evidence of primitive life. Imagine a giant meteorite plummeting into Mars four billion years ago, knocking chunks of the planet into space, where they circled for countless millenia. Imagine one of these space rocks finally colliding with Earth. But instead of wiping out the dinosaurs, this meteorite dropped off the life forms that created them.

Of course, the study of impossibly ancient extra-terrestrial microorganisms is not a well developed science. Meteorite ALH 84001 remains the focus of intense debate, and the notion of planets seeding other planets remains only a theory.

Even if life on Earth did evolve from life on Mars, that still doesn’t tell us how life evolved on Mars. Perhaps life evolved on both planets, which also exchanged rocks bearing primitive space travelers.

Space Travelers

Some people have suggested that Earth might have been deliberately stocked with life by advanced beings from another solar system or even galaxy.

The late astronomer Carl Sagan didn’t think that was likely. He questioned whether even the most advanced life form could navigate the impossibly vast distances of deep space.

Creation

The major Western religions teach that life was created by a being Christians call God. Eastern religions offer more complex accounts that are sometimes strikingly similar to the theory of evolution.

Some Christians have tried to make the creation story more acceptable by supporting it with pseudo-science, or “junk science.” In recent years, Intelligent Design (ID) has become a hot topic for debate. ID is essentially creationism supported by more sophisticated junk science and more corrupt media.

To understand what Intelligent Design is all about, consider its source: Seattle’s Discovery Institute. This bizarre organization’s Board of Directors includes some of America’s most corrupt corporate executives and politicians. It is associated with Microsoft and other corrupt corporations.

The Discovery Institute will be covered in more detail in Kids’ Politix. GeoZoo focuses on more intelligent life, from mammals to sea slugs. :)