Origin of life on earth
Darwin’s theory of evolution tells us how life evolved from simple to more complex forms
and Mendel’s experiments give us the mechanism for the inheritance of traits from one
generation to the next. But neither tells us anything about how life began on earth in the
first place.
150 Science
Q U E S T I O N S
? 1. What are the different ways in which individuals with a particular
trait may increase in a population?
2. Why are traits acquired during the life-time of an individual not
inherited?
3. Why are the small numbers of surviving tigers a cause of worry from
the point of view of genetics?
9.4 SPECIATION
What we have seen so far is micro-evolution. That means that the changes
are small, even though they are significant. Also, they simply change the
common characteristics of a particular species. But this does not properly
explain how new species come into existence. That can be said to have
happened only if this group of beetles we are thinking about, splits into
two populations that cannot reproduce with each other. When this
happens, they can be called two independent species. So, can we extend
the reasoning we have used above to explain such speciation?
Consider what would happen if the bushes the beetles feed on are
spread widely over a mountain range. The beetle population becomes
very large as a result. But individual beetles feed mostly on a few nearby
bushes throughout their lifetime. They do not travel far. So, in this huge
population of beetles, there will be sub-populations in neighbourhoods.
Since male and female beetles have to meet for reproduction to happen,
most reproduction will be within these sub-populations. Of course, an
occasional adventurous beetle might go from one site to another. Or a
beetle is picked up by a crow from one site and dropped in the other site
without being eaten. In either case, the migrant beetle will reproduce
with the local population. This will result in the genes of the migrant
beetle entering a new population. This kind of gene flow is bound to
J.B.S. Haldane, a British scientist (who became a citizen of India later), suggested in
1929 that life must have developed from the simple inorganic molecules which were
present on earth soon after it was formed. He speculated that the conditions on earth
at that time, which were far from the conditions we see today, could have given rise to
more complex organic molecules that were necessary for life. The first primitive
organisms would arise from further chemical synthesis.
How did these organic molecules arise? An answer was suggested by the experiment
conducted by Stanley L. Miller and Harold C. Urey in 1953. They assembled an
atmosphere similar to that thought to exist on early earth (this had molecules like
ammonia, methane and hydrogen sulphide, but no oxygen) over water. This was
maintained at a temperature just below 100°C and sparks were passed through the
mixture of gases to simulate lightning. At the end of a week, 15% of the carbon (from
methane) had been converted to simple compounds of carbon including amino acids
which make up protein molecules. So, can life arise afresh on earth even now?
Darwin’s theory of evolution tells us how life evolved from simple to more complex forms
and Mendel’s experiments give us the mechanism for the inheritance of traits from one
generation to the next. But neither tells us anything about how life began on earth in the
first place.
150 Science
Q U E S T I O N S
? 1. What are the different ways in which individuals with a particular
trait may increase in a population?
2. Why are traits acquired during the life-time of an individual not
inherited?
3. Why are the small numbers of surviving tigers a cause of worry from
the point of view of genetics?
9.4 SPECIATION
What we have seen so far is micro-evolution. That means that the changes
are small, even though they are significant. Also, they simply change the
common characteristics of a particular species. But this does not properly
explain how new species come into existence. That can be said to have
happened only if this group of beetles we are thinking about, splits into
two populations that cannot reproduce with each other. When this
happens, they can be called two independent species. So, can we extend
the reasoning we have used above to explain such speciation?
Consider what would happen if the bushes the beetles feed on are
spread widely over a mountain range. The beetle population becomes
very large as a result. But individual beetles feed mostly on a few nearby
bushes throughout their lifetime. They do not travel far. So, in this huge
population of beetles, there will be sub-populations in neighbourhoods.
Since male and female beetles have to meet for reproduction to happen,
most reproduction will be within these sub-populations. Of course, an
occasional adventurous beetle might go from one site to another. Or a
beetle is picked up by a crow from one site and dropped in the other site
without being eaten. In either case, the migrant beetle will reproduce
with the local population. This will result in the genes of the migrant
beetle entering a new population. This kind of gene flow is bound to
J.B.S. Haldane, a British scientist (who became a citizen of India later), suggested in
1929 that life must have developed from the simple inorganic molecules which were
present on earth soon after it was formed. He speculated that the conditions on earth
at that time, which were far from the conditions we see today, could have given rise to
more complex organic molecules that were necessary for life. The first primitive
organisms would arise from further chemical synthesis.
How did these organic molecules arise? An answer was suggested by the experiment
conducted by Stanley L. Miller and Harold C. Urey in 1953. They assembled an
atmosphere similar to that thought to exist on early earth (this had molecules like
ammonia, methane and hydrogen sulphide, but no oxygen) over water. This was
maintained at a temperature just below 100°C and sparks were passed through the
mixture of gases to simulate lightning. At the end of a week, 15% of the carbon (from
methane) had been converted to simple compounds of carbon including amino acids
which make up protein molecules. So, can life arise afresh on earth even now?
