First Steps
There are two major groups of animals those
which have backbones and those which do not.
Animals with backbones are known as vertebrates.
The first vertebrates that developed were fish, and for a long time,
they were the only vertebrates.
Then, around 374 million years ago, some of these fish moved out of the sea
and began living on land.
These became the first “tetrapods.”
A tetrapod is a creature which has four limbs legs and either wings or arms,
depending on the kind of animal and a backbone.
Examples of tetrapods include reptiles, birds, and mammals such as human beings.
The movement of vertebrates from the sea to land is
considered to be one of the most important events in the history of life on Earth.
Even today, though, little is known about exactly how this occurred.
One reason for this is that relatively few fossils remain
from the time when fish were evolving into tetrapods.
The recent discovery of a complete fossil of an ancient fish in Canada,
however, has provided new hints about how this change might have happened.
The fossil is of a 1.6meterlong fish called an elpistostegalian.
Scientists have believed for some time that these fish,
which looked a bit like crocodiles and lived near the coast,
were one of the ancestors of tetrapods.
The fish had four fins, two at the front and two at the back,
and these may have developed into the four limbs of a tetrapod.
The discovery of a complete fossil in Miguasha National Park in Quebec, Canada,
has allowed scientists to examine the front fins of an elpistostegalian
for the very first time.
The scientists found that the front fins of this ancient fish contained
bones
like the ones in the hands of land animals.
Normally, fins do not contain any bones at all.
The scientists believe these bones developed to allow the fish to support its body
when it was in shallow water.
In other words, the fish began developing hands
and feet even before it left the sea.
This makes it even more likely that the elpistostegalian is
one of the links between fish and tetrapods. |
英語検定 ２級(2021年第3回No.4)