What is LIFE?

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What is the meaning the Life?

Other Properties of Life

Living things reproduce themselves. Either individually or in sexual pairs, they have both the encoded instructions and the machinery necessary for self-reproduction. (Some creatures cannot reproduce, but every creature comes from reproduction.) Periodic crystals like sodium chloride (table salt) also undergo a kind of self-reproduction. In crystals however, the "instructions" are much simpler, they are not encoded, and they are not different from the "machinery."
Life uses processes collectively called metabolism to convert materials and energy for its needs. Metabolism creates waste products. When metabolism ceases with no prospect of starting again, we call it death. Machines also convert materials and energy for their needs, create waste, and could be said to die.
Life undergoes evolution. Notably, simpler forms are succeeded by forms with greater organization. Cars evolve also, in their way. Computers do, too. And computers even contain their own encoded instruction sets.

What Is Motor Vehicle Traffic?

It is tempting to say that motor vehicle traffic is simply the things that move along the streets and highways — cars and trucks. Of course buses and motorcycles should be included, although they are absent or prohibited on some streets. But what about wheelchairs, bicycles and skateboards — sometimes these are motorized. What about a trailer that is merely towed behind a tractor? What about a tire that happens to come off and roll a tenth of a mile? What about rocks that fall out of a dump truck and bounce and skid along the highway?

As it turns out, motor vehicle traffic is quite difficult to define. But naturally it would be hard to draw a line between cars and trucks, and the bouncing and skidding rocks from which they must have evolved.

These latter properties of life are sometimes used to make the point that life is hard to define. But nothing else has all of these latter properties except cellular life using life's DNA—RNA—protein operating system. Another kind of life, entirely different from ours, is conceivable, yes. But the only kind we have ever seen is the one we are part of here on Earth. As biologist and philosopher Harold J. Morowitz says, "The only life we know for certain is cellular..." (4).

Viruses and prions are not alive; they lie on the fringe of life. Viruses contain instructions encoded in DNA or RNA. (Prions don't.) Both are reproduced. Viruses certainly and prions probably can evolve. But neither can reproduce itself; each needs the machinery of a living cell to carry out its reproduction. Without a cell, viruses and prions are merely inert, complicated particles which do nothing. Do they make it hard to define life? No, just as trailers don't make it hard to define motor vehicle traffic. We know what motor vehicle traffic is. And we know what life is.

A Cell Is Like a Computer

All the regularities of biology strike me as being exactly like the regularities of engineering — Daniel C. Dennett (4.5)

One analogy for a cell is a computer. Computers have coded instructions inside them called programs. The programs in computers are analogous to the genetic programming in the DNA within cells. DNA is subdivided into functional units called genes; these would correspond to files in the computer. A computer even has a metabolism: it consumes electrical energy and discharges heat.

The programs in cells and those in computers can both be 1) copied and 2) executed. Some of the proteins made when a genetic program is executed would loosely correspond to the computer's paper printout. But other proteins are more analogous to the computer's cabinetry or wiring. Of course, computers don't make their own cabinetry or wiring; the analogy is not perfect.

In fact, nothing about the computer is analogous to a cell's reproduction. A cell can make a complete copy of itself; it contains the complete instructions (programs) and the cellular machinery (hardware) necessary to reproduce itself. A computer cannot make a copy of itself. It lacks the necssary machinery (but it may be able to reproduce its instruction set by "automatic full backup".) A computer that could reproduce itself would be more properly described as a self-reproducing robot. Such a thing is conceivable, but none exists on Earth today.

A multicelled creature is like a network of computers. It requires parallel computer architecture on a huge scale to operate multicelled creatures such as mammals with billions or trillions of cells, all working in harmony, each doing its task. The nervous system and the hormonal system are two important networking systems used by mammals.

Changing the way a computer works requires new programs. Sometimes one can simply insert a disc into a slot: the computer recognizes the disc, accepts its new code, and uses it. Other times, reprogramming a computer is more trouble. The new software may have "bugs"; it may not be compatible with the existing software; additional software patches may be needed; it may introduce a computer virus; or it may cause everything to crash without explanation.

Biological evolution happens when cells are reprogrammed. Somehow, new genetic programs are installed and activated. How does new genetic software get installed and activated? And where does it come from? These are some of the questions that Cosmic Ancestry attempts to answer.

The Two Kinds of Cells

There are two kinds of cells. You might guess the two are plant and animal cells. This distinction, however, is even more profound. The two kinds are prokaryotes and eukaryotes. (All plant and animal cells are eukaryotic.)

bacteria / Susan M. Barns

Prokaryotes are smaller and simpler than eukaryotic cells. They have no cell nucleus. They can multiply faster than eukaryotic cells, mainly for two reasons: 1) They have shorter genetic instructions to be replicated; and 2) The replication process goes about ten times as fast. Prokaryotes don't combine and specialize to form multicelled creatures. Prokaryotes are also called bacteria. They come in a wide variety of types; their diversity is much greater than that of eukaryotes.

Prokaryotes were here first, appearing very soon after Earth had cooled enough for life to survive. The oldest rocks that could contain recognizable fossils contain evidence of domelike structures left by colonies of cyanobacteria and other bacteria. Even older rocks contain chemical evidence that the metabolism of these bacteria was under way (5).

Prokaryotes are divided into two major subkingdoms: eubacteria and archaebacteria. Eubacteria, or "true bacteria", are more familiar and ubiquitous, thriving in soil, water, our own mouths, etc. Archaebacteria differ from eubacteria in some basic ways. For example, their ribosomes (nanoscale protein factories) have a different shape. In fact, archaebacteria are in some ways more similar to eukaryotes than to eubacteria. Biologists now think, based on the reconstruction of genetic "trees," that archaebacteria are the oldest kind of cell. Today some biologists maintain that archaebacteria constitute a third domain of life which could be called simply archaea (6-8).

There are four types of archaea. Two are known for their ability to inhabit extremely hostile environments such as very salty brines, and boiling springs or ocean thermal vents. The third group of can metabolize some very unappetizing chemicals to make methane. A fourth type, the sulfate-reducers, were recently distinguished from the others (9).

eukaryote / The ESG Biology Hypertextbook

Eukaryotic cells are much more complicated than prokaryotic cells. The eukaryotic cell has a differentiated nucleus enclosed in a nuclear membrane. It usually has two whole copies of the genome, so in computer terms the eukaryotic cell has a backup copy of its programs. Outside of its nucleus, the eukaryotic cell has an array of complex subunits that are essential to it. Two of the subunits, mitochondria and plastids, have their own DNA. These two subunits enable eukaryotic cells that contain them to conduct respiration and photosynthesis, respectively. Eukaryotic cells are able to constitute multicelled animals and plants. Eukaryotes are able to acquire much more complex features than prokaryotes. If life has existed on Earth for almost four billion years, the consensus is that eukaryotes first appeared just after the halfway point, maybe 1.7 billion years ago.

Returning to the computer analogy, the relationship between prokaryotes and eukaryotes is like the relationship between handheld calculators and desktop personal computers. Both kinds of cells come in a broad range of sizes. Prokaryotes are, on average, about an order of magnitude smaller, like handheld calculators. And they come in a wide variety, each with a narrow special purpose. Consider scientific calculators, inventory scanners, GPS units, cellphones, cordless phones, pagers, beepers, walkie-talkies, PDAs, TV remote controllers, keyless entry buttons, Gameboys, Walkmans, iPods, guitar tuners, electronic or medical diagnostic kits, digital cameras, smoke detectors, portable radios, digital thermometers and cordless shavers. Like eukaryotes, personal computers have greater memory capacity, have more complicated structure, and can be networked (eukaryotes form multicelled creatures).

The size of a cell's genome can be compared to the amount of programming stored in a computer, using the equation, 4 nucleotides = 8 bits = 1 byte. The simplest prokaryotic cell would correspond to a handheld calculator with about 200 kilobytes of stored programs; the E. coli bacterium would correspond to a handheld calculator with about 1.2 megabytes of stored programs. Among eukaryotic cells, counting the backup copy of the genome and the "silent" DNA, a yeast cell would correspond to a personal computer with 12 megabytes of program storage capacity; a human cell corresponds to a personal computer with 1.5 gigabytes of program storage capacity. And the human body would correspond to a computer network of a hundred trillion (10^14) or more such units.

Life in Science

There is no universally accepted scientific definition of life, but most definitions are a variation on: life is the “condition that distinguishes animals, plants, and other organisms from inorganic or inanimate matter, characterized by continuous metabolic activity and the capacity for functions such as growth, development, reproduction, adaptation to the environment, and response to stimulation” (Oxford English Dictionary). Thus scientifically speaking animals, plants, and bacteria are all examples of living things, as is a liver cell in an incubator.

This standard biological characterization of life is of course at odds with how the word life is generally used in the abortion debate. Discussions on abortion are often anchored by the question of when life begins. For example, the recent comments of the senatorial candidate Richard Mourdock implicitly reflected this in his statement “And even when life begins in that horrible situation of rape, …”. Indeed, the official position of the Catholic Church is that life begins at conception, that is, that the act of a sperm fertilizing an egg creates life. A sperm is a living cell, an egg is a living cell, and the union of a sperm and an egg creates another living cell: the zygote. In the biological sense of the word, life is certainly not created by the act of fertilization; rather fertilization is one of the many steps in the perpetuation of organisms that undergo sexual reproduction. Nevertheless, a zygote is a very special cell indeed: it is the only cell that, under the right circumstances, has the potential to develop into a complete and entirely unique human being.

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Of course, as the reader has probably already noted, when most people speak of when life begins they are not actually using the term life in the scientific sense of the word. They mean when the life of a new human being begins. So the abortion issue pivots on the question: at what point in development does the collection of cells that form the fetus become a new human being? The notion of being human is generally inextricably intertwined with being conscious—or at least of having the capacity for consciousness (I am not conscious while anesthetized on a surgery table, but I clearly retain the capacity for consciousness and thus for humanness). This is generally true in the legal context as well, which is why a human body with a beating heart is considered to be legally dead (no longer a human) if there is no brain activity. If one agrees to use consciousness, or the capacity for consciousness, as the benchmark for when a human life begins, then we can make definite statements as to when a fetus cannot be considered to yet be a human being—but making definite statements as to when a fetus is a human being is much more challenging. A fetus cannot be conscious if it does not have a functional brain. Thus, a minimal set of developmental landmarks for consciousness to arise must include the presence of functional neurons—that is neurons that transmit information to each other. An additional condition is that the cortex (the more massive and evolutionarily recent part of the brain) must receive signals from the thalamus (one of the “hubs” that interfaces information between the body and the cortex). While there is ongoing debate as to exactly when functional connections between the thalamus and cortex arise it is certainly not before three months gestation1,2. Thus it can be stated with a high degree of certainty that at three months a fetus cannot be conscious.

The ongoing debate about the ethics of abortion is both healthy and necessary. But the debate will be futile if there is no attempt to use science to define what it means to be human. The task is a challenging one because it relates to two of the most profound and complex questions mankind has ever asked: what is life and what is consciousness.

There is an additional, rarely articulated, impediment to a more rational debate about abortion: our reticence to accept that our humanity, our ability to think, create, learn, laugh, and love, emerges from the physical brain. This is a concept that most people are uncomfortable with. But our reticence to acknowledge that our humanity derives solely from the brain should not come as a surprise, because this reticence is itself a product or how our brain works. The fact that something as intimate and powerful as our own consciousness arises from the conglomerate of 100 billion interconnected neurons incased within our skulls is among the most counterintuitive idea humans have ever faced. But it is counterintuitive simply because the brain did not evolve to understand itself any more than a computer was designed to write its own operating system.

For more on the how the brain works, and its flaw and limitations.

What is life?