Read Online What a Wonderful World by Marcus Chown - Free Book Online Page B
Authors: Marcus Chown
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eukaryotic cells bumped into each other and
accidentally fused
. Now, some cells are known to shift to a dormant state, barely ticking over, when times are tough – for instance, during a drought. At such a time, a cell consisting of two cells fused together might have a survival advantage. After all, two cells will have
pooled their resources
. And this may not be the only advantage of the fused cell. The tough time may be tough enough actually to damage the cell’s DNA. But, since the cell has two copies of its genes, it has the ability to compare the two copies and
correct any errors
.
When the good times return, a cell with only one copy of its genes will have an advantage once again. After all, with less DNA to copy, it will be able to reproduce more quickly and proliferate. This may therefore have driven the evolution of meiosis, the means of creating cells with only one copy of their genes. If this seems implausible, there are indeed single-celled organisms today that react to extreme changes in their environment by switching back and forth between a state with one copy of their genes – known as haploid – and one with two copies – known as diploid.
So much for how cells came to fuse and then unfuse in the process of meiosis. How did the DNA of two cells intermingle to create the genetic variety so central to sex? It turns out that this happens naturally in the process of repairing damaged DNA. When a cell detects a difference between the two complementarystrands of DNA on a chromosome, it has no idea which strand is error-free. It therefore has no choice but simply to excise the region from both strands of DNA. This leaves a gap, which the cell fills by copying the sequence present at the same region on the matched chromosome.
All this happens when the two chromosomes are very close together. And, crucially, in the complex dance – which involves cutting up bits of DNA physically and touching them together – bits of DNA get swapped around. This process, known as crossover, ensures that, when the meiosis creates new cells, each is different from its parent. It is a happy accident that might have become frozen because natural selection favours organisms whose offspring are novel and varied.
So, it seems, sex was a simple accident that evolved into a survival strategy. It made use of pre-existing genes. Nature was left with a mechanism that unintentionally mixed DNA, greatly boosting genetic variation, causing the rate of evolution to explode.
But, of course, there is a lot more than this to sex between complex organisms such as human beings. How did
that
evolve? Nobody knows the precise details. However, it is possible to speculate on the steps along the road. First there was the evolution of cells that could fuse together and undergo meiosis. This was the origin – the big bang – of sex. Next came the evolution of sexes. Rather than a single type of cell, there arose two kinds: male and female. 19 At first, the two types were able to fuse together in all possible combinations: male–male, female–male and female–female. However, the combining of different types, or outbreeding, creates more genetic variation among offspring, which has survival advantages. Eventually, therefore, a systemof sex evolved in which the only combination of cells that was viable was male–female.
In the beginning, all the cells of a sexually reproducing organism were capable of doing the deed. However, the next step in the evolution of sex was the advent of multicellular organisms in which sexual reproduction was down to
only one type of specialised cell
. One of the two types of gamete, known as sperm, evolved the ability to swim about, boosting its chance of finding the second type, known as the egg. But this was not the end of the specialisation. Eventually, the production of gametes was confined to only one type of the tissue: namely, the gonads.
Nobody knows how long all this took. But, evolution by natural
accidentally fused
. Now, some cells are known to shift to a dormant state, barely ticking over, when times are tough – for instance, during a drought. At such a time, a cell consisting of two cells fused together might have a survival advantage. After all, two cells will have
pooled their resources
. And this may not be the only advantage of the fused cell. The tough time may be tough enough actually to damage the cell’s DNA. But, since the cell has two copies of its genes, it has the ability to compare the two copies and
correct any errors
.
When the good times return, a cell with only one copy of its genes will have an advantage once again. After all, with less DNA to copy, it will be able to reproduce more quickly and proliferate. This may therefore have driven the evolution of meiosis, the means of creating cells with only one copy of their genes. If this seems implausible, there are indeed single-celled organisms today that react to extreme changes in their environment by switching back and forth between a state with one copy of their genes – known as haploid – and one with two copies – known as diploid.
So much for how cells came to fuse and then unfuse in the process of meiosis. How did the DNA of two cells intermingle to create the genetic variety so central to sex? It turns out that this happens naturally in the process of repairing damaged DNA. When a cell detects a difference between the two complementarystrands of DNA on a chromosome, it has no idea which strand is error-free. It therefore has no choice but simply to excise the region from both strands of DNA. This leaves a gap, which the cell fills by copying the sequence present at the same region on the matched chromosome.
All this happens when the two chromosomes are very close together. And, crucially, in the complex dance – which involves cutting up bits of DNA physically and touching them together – bits of DNA get swapped around. This process, known as crossover, ensures that, when the meiosis creates new cells, each is different from its parent. It is a happy accident that might have become frozen because natural selection favours organisms whose offspring are novel and varied.
So, it seems, sex was a simple accident that evolved into a survival strategy. It made use of pre-existing genes. Nature was left with a mechanism that unintentionally mixed DNA, greatly boosting genetic variation, causing the rate of evolution to explode.
But, of course, there is a lot more than this to sex between complex organisms such as human beings. How did
that
evolve? Nobody knows the precise details. However, it is possible to speculate on the steps along the road. First there was the evolution of cells that could fuse together and undergo meiosis. This was the origin – the big bang – of sex. Next came the evolution of sexes. Rather than a single type of cell, there arose two kinds: male and female. 19 At first, the two types were able to fuse together in all possible combinations: male–male, female–male and female–female. However, the combining of different types, or outbreeding, creates more genetic variation among offspring, which has survival advantages. Eventually, therefore, a systemof sex evolved in which the only combination of cells that was viable was male–female.
In the beginning, all the cells of a sexually reproducing organism were capable of doing the deed. However, the next step in the evolution of sex was the advent of multicellular organisms in which sexual reproduction was down to
only one type of specialised cell
. One of the two types of gamete, known as sperm, evolved the ability to swim about, boosting its chance of finding the second type, known as the egg. But this was not the end of the specialisation. Eventually, the production of gametes was confined to only one type of the tissue: namely, the gonads.
Nobody knows how long all this took. But, evolution by natural
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