Diversification of Life

Life on earth started in the form of simplest unicellular (prokaryotic) microorganisms. In course of time, these organisms evolved to utilize solar energy through chemical process called photosynthesis. It is through the photosynthetic activity of the earliest autotrophs that oxygen built up gradually in the earth’s atmosphere making it possible for complex heterotrophic organisms to evolve.

For a very long time (nearly 3 GY) after the origin of life, earth had no life forms other than prokaryotes (cells lacking nucleus) comprising different groups of bacteria. There were neither plants nor animals. Eukaryotes (cells with nucleus) probably appeared about a billion years ago, but life was mostly in the form of unicellular (single celled) organisms.

Then suddenly, about 600 million years ago, in a geological period called Cambrian, there was a great, almost explosive, diversification of life into multicellular organisms with a variety of body plans and life styles. Biologists call this period the Cambrian explosion.

Fossils, the remains of plants, animals and lower living beings provide evidence for the sequence in which different kinds of living organisms came to exist on the earth.

When a fossil is collected, the age of the sedimentary rock in which it is found is determined and that age is generally taken as the time in earth’s history when that particular animal lived. Paleontologists (Scientists who study fossils) are able to reconstruct the history of life on earth from the fossils collected in sedimentary rocks of different ages. They clearly show that species and higher taxonomic groups (like angiosperms, insects and birds) evolved gradually.

Diversity of Life Resulted from Evolution

When we explore nature, we observe that

  1. There is so much diversity of microbes, plants and animals in the biosphere of our planet.

  2. Many animals and plants share common features. We humans are similar to rats, horses, elephants and tigers in possessing hair and mammary glands. Further, we share features like vertebral column with birds, snakes, frogs and fishes. In fact, all living organisms have so many characteristics in common, including DNA, the hereditary molecule.

  3. There is so much variation even among individuals of the same species.

These three observations lead us to ask important questions. How and why did such huge diversity of life forms arise? Were the diverse life forms present from the beginning of earth’s history or did they arise gradually over a period of time? Why do even remotely related organisms have so many features in common? Is it possible because they all came from a single ancestor? Why is there so much variation within any single species?