LIFE ON EARTH

Life on Earth Mechanism and vitalism Human beings are ambulatory collections of some 1014 cells. Human cells are in many fundamental respects similar to those that make up all the other animals and plants on the Earth. Each cell typically consists of a central, usually spherical, nucleus and an outer more heterogeneous region, termed the cytoplasm. The substance of nucleus and cytoplasm together has for many decades been called protoplasm. Use of this term implied that there was some special substance underlying living organisms. In the use of the word protoplasm there is occasionally an implication that life cannot be explained solely by physics and chemistry, that some mysterious vital force must be invoked. A living cell is a marvel of detailed and complex architecture. Seen through a microscope there is an appearance of almost frenetic activity. On a deeper level it is known that molecules are being synthesized at an enormous rate. Almost any enzyme catalyzes the synthesis of more than 100 other molecules per second. In 10 minutes, a sizable fraction of the total mass of a metabolizing bacterial cell has been synthesized. The information content of a simple cell has been estimated as around 1012 bits, comparable to about a hundred million pages of the Encyclopdia Britannica. Faced with all this or its equivalent, it is not surprising that early biologists felt despair at ever being able to understand the detailed workings of life. A Stone Age man, confronted for the first time with a watch, might also deduce that there was some special watch substance in nature, or perhaps even a god of the watch. In ancient times, the most common of biological activities, such as the hatching of an egg or the blooming of a flower, were attributed to the intercession of a deity. After the epochal work of Sir Isaac Newton, when the motion of the planets and comets of the solar system was predictable to some very great precision and understood on the basis of an underlying principle, the idea developed that organisms were also nothing more than a particularly intricate kind of clockwork. But when early investigations failed to unveil the clockwork, a kind of ghostly mainspring was inventedthe vital force. This force was a rebellion from mechanistic biology, an explanation of all that mechanism could not explain or for which mechanism could not be found. It also appealed to those who felt debased by the implication that they were nothing more than a collection of atoms, that their urges and apparent free wills arose merely from the interaction of an enormously large number of molecules in a way that, although too complex to use predictably, was in principle determined. Not only is there no evidence for a vital force but the idea itself is hardly thought out; it is a sort of catchall concept, covering anything otherwise inexplicable. The alternative approach, that all organisms are made of atoms and nothing else, has proven especially useful and has led to a fundamental new understanding of biological systems. This situation does not imply, of course, that atoms cannot be put together in so complex a way that their collective behaviour is too difficult to understand in terms of the individual atoms; in this sense there may be particular laws of biology not readily derivable from the elementary interaction of atoms. But this is a very different thing from a vital force. Indeed, there is nothing debasing in the thought that a person is made of atoms alone; it means that one is intimately connected with the matter that comprises the inanimate universe. What a wonder that atoms can be put together in so complex a pattern as to produce human beings. Man is a tribute to the subtlety of matter. As the American anthropologist Loren Eiseley has written, . . . if dead' matter has reared up this curious landscape of fiddling crickets, song sparrows, and wondering men, it must be plain even to the most devoted materialist that the matter of which he speaks contains amazing, if not dreadful powers. . . . (The Immense Journey, Random House, New York, 1957.) Nucleic acids It is now known that many if not all of the fundamental properties of cells are a function of their nucleic acids, their proteins, and the interactions among these molecules. Within the nuclear regions of cells is a mlange of twisted and interwoven fine threads, the chromosomes. During cell division, in all but the simplest organisms, the chromosomes display an elegantly choreographed movement, separating so that each daughter cell of the original cell receives an equal complement of chromosomal material. This pattern of segregation corresponds in all details to the theoretically predicted pattern of segregation of the genetic material implied by the fundamental genetic laws (see heredity). The chromosomes are composed of nucleic acids and proteins in a combination called nucleoprotein. The nucleic acid stripped of its protein is known to carry genetic information and to regulate cellular metabolism; the protein in nucleoprotein undoubtedly plays some secondary, probably regulatory, role. The specific carrier of the genetic information in higher organisms is a nucleic acid known as DNA, short for deoxyribonucleic acid. DNA is a double helix, two molecular coils wrapped around each other and chemically bound one to another by bonds connecting adjacent bases. Each helix has a backbone that consists of a long sequence of alternating sugars and phosphates. Attached to each sugar is a base. Each sugar-phosphate-base combination is called a nucleotide; a nucleic acid strand can be thought of as a sequence of nucleotides. There is a very significant one-to-one base pairing in the connection of adjacent helices, in the sense that once the sequence of bases along one helix is specified, the sequence along the other is also specified. The specificity of base pairing plays a key role in the replication of the DNA molecule, where each helix makes an identical copy of the other from molecular building blocks in the cell. These nucleic acid replication events are mediated by enzymes, and with the aid of enzymes have been produced in the laboratory. Ribonucleic acid (RNA) differs from DNA in having a slightly different five-carbon sugar, and in replacing one of the four bases that make up DNA by a slightly different base. RNA does not appear to exist in a double-stranded form. Now DNA, RNA, and the enzymes have a curiously interconnected relation, which appears ubiquitous in all organisms on Earth today.