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Where Did Life Come From?

Where Did Life Come From?
u Based on the fossil record and genetic evidence, we believe that life began on Earth approximately 4 billion years ago. The first life-form is called the last universal common ancestor. It was the first thing that we can say wasn’t just a self-replicating molecule and was actually a living organism that evolved into all of the organisms that have ever existed on Earth.

Where Did Life Come From?
u There have been many different kinds of life since then. We think there have been more than 1 billion distinct species on Earth through its entire history and that more than 98% of those species are now extinct. In fact, we don’t really know how many different species of animals are alive on Earth today. We’ve catalogued around 1.5 million species, but we estimate that the number we haven’t catalogued or even discovered range from another million to another 7 million.
u The first forms of life were what zoologists call protocells. They each had a membrane and cytoplasm and a number of functional structures in that cytoplasm working together to perform the most basic process we attribute to living things: self-replication.
u At some point fairly early on in the evolution of life, the nucleic acids came into being. We could say that this was the moment that life as we know it emerged, because now our common ancestor was self-replicating and passing on its characteristics to its offspring by means of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
u When it comes to building an organism, DNA is the blueprint and RNA is the construction worker. The single-helical RNA molecule reads the instructions for protein building that are encoded in the double-helical DNA molecule, then takes those instructions from each cell nucleus—where the DNA lives—to the other structures in the cytoplasm of the cell, where all other chemical reactions necessary for life take place.
u When it comes to cell division, or mitosis, the DNA replicates itself. When the replication is complete, the nucleus splits itself in 2, then the cytoplasm splits itself in 2, and 1 cell becomes 2 cells.
u Sometimes the replication process doesn’t go right, and the DNA of the new cell doesn’t quite match the DNA of the old cell. These mistakes are called mutations.
u Human beings have 23 pairs of chromosomes—that is, molecules of DNA—in every one of their somatic cells. That amounts to about 6 billion individual nucleotides—6 billion bits of information per cell that have to be copied every time one of your cells divides. There are bound to be mistakes.
u Mutation is an important natural process. It’s the process by which evolution occurs.