How life is made

There are 46 chromosomes in the living cell of a human being and these chromosomes carry the genetic information that decides how a person will grow - whether he or she will be dark or fair, short or tall, blue-eyed or brown. But the sex cells, the female egg and the male sperm, each have only 23 chromosomes. They fuse at conception to make a cell containing 46 chromosomes, half from each partner, and it is this mixing of two sets of characteristics that creates the diversity of human life. Other living things which reproduce sexually also have chromosomes, but the numbers vary widely from species to species. A garden pea, for instance, has 14 chromosomes, a potato 48 and a crayfish 200.

In 1953, an American scientist, Stanley Miller, attempted to reproduce the atmosphere and weather conditions of Earth at the very dawn of life. He passed an electric charge through a mixture of hydrogen, methane and ammonia gases for 20 hours in an enclosed water bath. In the process, he formed some organic compounds, including amino acids, the basic building material of living cells. In 1979 at the University of Texas, researchers Allen J. Bard and Harald Reiche produced amino acids by exposing to sunlight a solution of ammonia, methane and water containing particles of platinum and titanium oxide. These experiments in man-made life suggest that life began as the result of the Sun's rays and violent electrical storms acting on the gases present in the young Earth's atmosphere.

Stunted bristlecone pines in the arid White Mountains of California - the world's oldest trees:- cling to a tenuous life and still grow very slowly in the occasional rainfall after 4600 years of life. But they are not the oldest living things on Earth. In sandstone rocks in a dry valley of Antarctica, American scientists have found tiny lichens- primitive partnerships of plant and fungus-which they have calculated to be at least 10,000 years old.

Biologists believe that the whole history of evolution is re-created in the embryo of an animal as it forms in its mother's womb. At first, the fertilised egg is a single cell like the simplest and earliest forms of life on Earth. But within hours. it begins to divide and multiply: into 2,4,8,16 cells and so on. For some time, all the cells produced by this process of division are very similar. Each cell has the capacity to become any one of the specialist cells in the adult creature. Then this cluster of undifferentiated cells gradually forms into a hollow ball containing two distinct layers, the inner endoderm and the outer ectoderm. A little later, a third layer, the mesoderm, develops between them. Once these changes take place, no cell can give rise to 'daughter' cells of the other two types.

As the embryo continues to develop and grow, its cells gradually become more and more specialised. The ectoderm cells become skin or nervous tissue, for instance. Endoderm cells can become parts of the digestive system or lungs. Mesoderm cells may develop into muscle and bone. At the same time, the embryo retraces the long evolutionary climb from simple to complex. A four-week-old human embryo, for example, has slits in the region of the neck which are like the gills of a fish. At six weeks, when the embryo is 12mm (0.5in) long, the shape is still that of a marine animal. but the slits, known as branchial arches. have begun forming into the upper and lower jaws. At seven weeks, the developing child has the attributes of a primate. It has a clearly visible tail and the arms are longer and stronger than the legs - proportions that will be reversed in the born human.

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