What Is Life? What'sNEW since 1999
I am not going to answer this question — J. B. S. Haldane (1)
The units of life are cells — Lynn Margulis (2)
Life Is CellsPeople like to say, as if it were obvious, that life is hard to define. This is misleading. Life has properties that clearly distinguish it from everything else. First, every living thing is cellular. In other words, it is either a single-celled creature or a creature composed of many cells. Every cell is bounded by its own outer membrane and contains a full set of instructions necessary for its operation and reproduction. Furthermore, every cell uses the same operating system: "DNA makes RNA makes protein." DNA is a long complex molecule that contains the cell's instructions. It is transcribed into RNA, another long complex molecule similar to DNA; and then the RNA transcript is translated into protein. There are hundreds of billions of different proteins used by living things (3), but all of them are made from the same twenty amino acids, the "building blocks of life."
Other Properties of LifeLiving 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.
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.(4.2)
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 millions of millions 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 CellsThere 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.)
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 bacterial metabolism 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 (6-9).
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, but 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 and life only: a radical alternative to life definitionism" by Carlos Mariscal and W. Ford Doolittle, doi:10.1007/s11229-018-1852-2, Synthese, 09 Jun 2018.
References1. J.B.S. Haldane, What Is Life?, New York: Boni and Gaer, 1947. p 53.
2. Lynn Margulis, Symbiotic Planet: A New Look at Evolution, Basic Books, 1998. p 69.
3. Martin Olomucki, The Chemistry of Life. McGraw-Hill, Inc., 1993. p 25.
4. Harold J. Morowitz, The Beginnings of Cellular Life: Metabolism Recapitulates Biogenesis, Yale University Press, 1992. p 12.
4.2. Sydney Brenner, "Life's code script," Nature, 22 Feb 2012.
4.5. Daniel C. Dennett, Darwin's Dangerous Idea: Evolution and the Meanings of Life, New York, Simon & Schuster, Inc., 1995.
5. J. William Schopf, "The Oldest Fossils and What They Mean" p 29-63, Major Events in the History of Life, J. William Schopf, ed., Jones and Bartlett Publishers, 1992.
6. C. R. Woese, O. Kandler and M. L. Wheelis, "Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya" [abstract], p4576-4579 v87, PNAS, June 1990.
7. Michael W. Gray, "The third form of life," p 299 v 383 Nature, 26 September 1996.
8. Hans-Peter Klenk, Lixin Zhou and J. Craig Venter, "Understanding life on this planet in the age of genomics," in Instruments, Methods, and Missions for the Investigation of Extraterrestrial Microorganisms, Richard B. Hoover, ed., Proceedings of SPIE Vol. 3111, p 306-317 (1997).
9. Carl R. Woese, and Norman R. Pace, "Probing RNA Structure, Function, and History by Comparative Analysis," p 91-117. The RNA World, R.F. Gesteland and J.F. Atkins, eds., Cold Spring Harbor Laboratory Press, 1993.
10. Leslie E. Orgel, The Origins of Life: Molecules and Natural Selection, John Wiley and Sons, Inc., 1973. p 92-93.
11. Denis Noble interviewed by Suzan Mazur, Huffpost, 2014.