We cannot have more than a speculative picture of the first living
creatures upon the earth or, rather, in the waters that covered the
earth. A basis for speculation is to be found, however, in the simplest
creatures living to-day, such as some of the bacteria and one-celled
animalcules, especially those called Protists, which have not taken any
very definite step towards becoming either plants or animals. No one can
be sure, but there is much to be said for the theory that the first
creatures were microscopic globules of living matter, not unlike the
simplest bacteria of to-day, but able to live on air, water, and
dissolved salts. From such a source may have originated a race of
one-celled marine organisms which were able to manufacture chlorophyll,
or something like chlorophyll, that is to say, the green pigment which
makes it possible for plants to utilise the energy of the sunlight in
breaking up carbon dioxide and in building up (photosynthesis) carbon
compounds like sugars and starch. These little units were probably
encased in a cell-wall of cellulose, but their boxed-in energy expressed
itself in the undulatory movement of a lash or flagellum, by means of
which they propelled themselves energetically through the water. There
are many similar organisms to-day, mostly in water, but some of
them--simple one-celled plants--paint the tree-stems and even the
paving-stones green in wet weather. According to Prof. A. H. Church
there was a long chapter in the history of the earth when the sea that
covered everything teemed with these green flagellates--the originators
of the Vegetable Kingdom.
On another tack, however, there probably evolved a series of simple
predatory creatures, not able to build up organic matter from air,
water, and salts, but devouring their neighbours. These units were not
closed in with cellulose, but remained naked, with their living matter
or protoplasm flowing out in changeful processes, such as we see in the
Amoebæ in the ditch or in our own white blood corpuscles and other
amoeboid cells. These were the originators of the animal kingdom. Thus
from very simple Protists the first animals and the first plants may
have arisen. All were still very minute, and it is worth remembering
that had there been any scientific spectator after our kind upon the
earth during these long ages, he would have lamented the entire absence
of life, although the seas were teeming. The simplest forms of life and
the protoplasm which Huxley called the physical basis of life will be
dealt with in the chapter on Biology in a later section of this work.