CREATION OF LIFE
AND BIOLOGICAL EVOLUTION ( Page 5) |
|
| Nearly 2 billion years passed after the first appearance
of life before advanced unicellular organisms, complete with nucleus
evolved. Though the pace of evolution was still very slow, the existing
varieties of life were multiplying fast and life was becoming abundant
in the oceans. But a real explosive increase in the number and variety
of life forms began with the next revolution of biological evolution,
the exchange of genes by the process of sexual reproduction. This
took place some 1.2 billion yeas ago. Still there were only single-celled
organisms and only in water. First fossil evidence of multicellular
organisms such as sponges and worms belong to a period some 600 million
years ago. This was followed by a profusion of aquatic life forms,
trilobites, clams and other invertebrates. In the plant kingdom, other
than plankton and diatoms there were algae of a few varieties. Sometime
along this long route to progress, somewhere about 500 million years
ago, some plant organisms had found their way into fresh water rivers,
lakes and swamps. |
| Out of the Oceans: |
| The oceans had nurtured life for more than 3 billion
years before life appeared on land or in fresh water. How could we
explain this? Why this delay? The answer is quite simple. Fresh water
lakes and rivers are found on landmasses or continents. There were
no such landmasses for the first 2 or 3 billion years. There were
only some archipelagos, groups of tiny islands, which were the peaks
of sub-oceanic mountains formed by the volcanic activity. The first
super continent – Pangea, to the geomorphologist – formed
around the South Pole about 600 million years ago. By now the atmosphere
took in the present composition and the earth became more hospitable
to life. Rains and seasons became decisive factors in the spread of
life. Aquatic life forms began to spread in fresh water streams, rivers
and swamps on this super continent. |
| Still the dry land was desolate. And then, some algae
and lichen that was on the edges of swamps slowly found their way
onto wetland near the swamps and gradually onto dry land. This, the
start of the colonization of the land took place some 500 million
years ago. Till a few years ago the general idea among the scientific
circles was that life forms from the ocean came to the land. But recently
it was confirmed that it was from fresh water that life came to the
land. From the simple plants like moss and lichens higher forms like
ferns and cycads evolved. Even much later the land became host to
the first true trees. But these were still, of the non-flowering variety.
Not even grass was there. The first forests were of the coniferous
trees like pines and firs. |
| Vertebrates |
| When these were happening on land some equally important
stage in the history of biological evolution was being enacted in
the oceans. This was the appearance of fish, the first vertebrate
organisms, those with internal skeleton with backbone as the mainstay.
We still have no idea as to when and how the first animals made the
appearance on land. But we know that the earliest of these were invertebrates
like spiders and dragonflies. One of the most interesting chapters
in the story of evolution of life that I have come across is the story
of how the fish came to walk on land. Like many other evolutionary
stories this one also has its origins in the fossil remains and fossil
footprints. Here the fossil and the fossil footprints belonged to
what the scientists call a tetrapod, a fish that had adapted four
of its fins modified to act as feet. This was discovered on the shores
of Ireland and dated some 450 million years old. It is said of palaeontologists,
that they make a mountain of theories out of a molehill of evidence.
Yet, often these theories hold water and help us to a better understanding
of the world around us. I personally liked the story built around
the fossil of the tetrapod as it gives us the most likely scenario
of how vertebrates came to land. The story goes like this. |
| A certain type of fish that made a swamp or a marshy
place its home found it hard to swim around in the thick soup of marsh
with the aid of the fins. It was easier to get along and find its
food by holding on to the floor and to the reeds and branches in the
swamp. For this purpose it adapted its fins for gripping and thus
slowly the fins turned to four legs. The tail fin stayed as a fin
as long as it was in water or marshy places. The shortage of oxygen
in the marsh made the fish to replace the gills by lungs. Soon it
found that it could find its food from the shores of the swamp as
well. Then one fine day it said good-bye to the water and came to
the land and became the first vertebrate to occupy land. It couldn’t
have been a mammal, most likely a reptile, as mammals were still higher
up in the scale of life. But it seems that in appearance this first
creature looked like something between a dog and a lizard. A very
surprising fact is that the link between this tetrapod and the fish
still exists today. In the marshes near the Congo River you find a
fish without fins. It is called the long fish. This has, in place
of the fins four long wire-like appendages. They use these limbs effectively
to move about in the thick muddy swamps. The tail fin tells you that
this is at least in part a fish. This species has developed some mechanism
for breathing atmospheric oxygen in cases of drought, along with dissolved
oxygen. This species has not undergone any evolutionary change during
the last 400 million years. This is one of what we call a living fossil.
|
|
