First Living Cells
This first live forebear of all living things, including Man himself, was formed by chance, probably over 3,500 million years ago. Scientists have shown that the building blocks of life – carbon-containing organic molecules – existed on Earth, and in space, soon after the planet’s formation 4,600 million years ago. They were in the gases that made Earth’s atmosphere, in the outpourings of volcanic eruptions, in the meteorites that pounded the young planet – and even in the dust of space itself.
Space probes by the US and former Soviet Union have confirmed that carbon molecules exist in the icy bodies and tails of comets, such as Halley’s comet, which periodically passed close to Earth. Astronomers have identified 75 different interstellar complex molecules that have come into being in the star-forming clouds of space. Living cells need protein molecules to form their structure, and DNA (deoxyribonucleic acid) molecules to hold information on how to develop. Both protein and DNA are formed from other molecules, called amino acids and bases. These consist of even simpler molecules. The odds against the correct combinations coming together by chance are enormous. Yet somehow, within the Earth’s first five thousand million years, they did.
Lighting bolts from electrical storms bombarded the Earth and its watery surface, where shallow pools lay rich with organic molecules – a warm soup of potential life. Massive doses of ultraviolet radiation from the sun poured down into the atmosphere and on to the surface, as yet unprotected by oxygen and an ozone layer. Great numbers of meteorites – some probably containing complex molecules such as amino acids – pierced the unresisting atmosphere and plummeted to Earth. Lava and hot gases poured out of volcanic fissures and spewed violently forth from eruptions, creating hot-spots in the atmosphere, and adding more gases to its composition.
Single cell life
Triggered by the energy from these violent natural events, the molecules in the primeval soup joined up to form complex chains. So a few hundred million years after the Earth’s formation, all the components of living cells were present in the ocean and it was only a matter of time before they were organized into cells. These came into being when various fats and protein joined together to form an outer membrane. This separated the contents of the cell from the water it floated in.
For over 2,000 million years, cells remained tiny and primitive, each hardly and thousandth of a millimetre in diameter. The first living cell split into two, each containing the DNA information necessary for further development. Two became four, and four became eight. Eventually those cells lived in teeming accumulations, millions to the cubic centimetre.
Cell mutations
As these cells reproduced, slight changes or mutations occurred in them. Sometimes these chance mutations were favourable and they helped the cell survive, often in conditions different from those in which the parent cell lived. Some cells eventually mutated into a life form that contained chlorophyll, the substance that makes plants green. With the assistance of chlorophyll, these cells started to make sugars using the energy of sunlight.
The sunlight – using cells needed hydrogen for energy conversion. They obtained it by splitting water molecules into their separate hydrogen and oxygen components. The cells used the hydrogen and released the oxygen as waste, Great matted masses of these oxygen gen-releasing cells lived in warm, shallow seas. After hundreds of millions of years, oxygen build up into a significant ingredient in the Earth’s atmosphere.
A new type of cell began to develop about 1,400 million years ago. Dependent on oxygen, this new cell was larger and much more complex. It had special nucleus for its DNA, and had at least a thousand times as much earlier, more primitive cells. Called a eukaryote, which means true nucleus, the new cell developed the ability to join up with other cells and share genetic information, so that their offspring’s had a much larger fund of genetic possibilities. Eukaryotes also became capable of forming multi-celled organisms, in which the individual cells have particular tasks. Nearly all life in today’s world is formed of eukaryote cells. The first fossils of multicellular life forms are 750 million years old
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