Question

Read the following statements and justify same in your own words with the help of suitable examples.
a. Human evolution began approximately 7 crore years ago.
b. Geographical and reproductive isolation of organisms gradually leads to speciation.
c. Study of fossils is an important aspect of study of evolution.
d. There is evidences of fetal science among chordates.

Answer 1

a. The dinosaurs became extinct around 7 crore years ago and from that point human evolution began. Humans are thought to have evolved from monkey like animals which looked more or less like modern lemurs. These monkey like animals further evolved into ape like animals. Some of these ape like animals evolved into gibbon and orangutan while rest of them evolved into gorilla and chimpanzees.From their evolved, the first human like animal called Ramapethicus. The following table lists the evolutionary history of modern man:

Year	Evolution
15 million years ago	Dryopithecus (ape-like) and Ramapithecus (man-like)
3 − 4 million years ago	Man-like primates
2 million years ago	Australopithecines, also called Homo habilis,lived in East Africa
1.5 million years ago	Homo erectus
1,000 − 40, 000 years ago	Neanderthal man
75, 000 − 10, 000 years ago	Homo sapiens

b. Speciation may be defined as an evolutionary process, which involves the formation of one or more species from an existing species. In order to understand speciation, let us consider the example of beetles. Let us consider that a population of beetles has split into two separate populations, which cannot reproduce with each other.

Formation of a new species

These two separate populations of beetles are spread on a wide mountain range since their food is widely distributed. Hence, the population of beetles in that area is very large. Beetles are small insects, which cannot travel to far off places. They gather food from nearby places. As a result, sub-populations of beetles are spread over that area. These sub-populations can lead to the formation of an entirely new species.

Geographical isolation: Since this population of beetles is spread over a large area, reproduction cannot occur between individuals of sub-populations. Reproduction will only occur within a sub-population, which will lead to the production of a new species. Now, if a river starts flowing between the two populations, then the two sub-populations would be further isolated and the chances of gene flow or reproduction further decreases.

Genetic drift and natural selection: Genetic drift and natural selection can give rise to different changes in sub-populations. For example, a particular sub-population of beetles evolves to blue or green colour due to natural selection or genetic drift. This will result in changes in subsequent generations. Thus, the two populations of beetles become completely different from each other.

These sub-populations will eventually be incapable of reproducing with each other. For example, the green female beetles of an area will prefer to reproduce with the green males only because green beetles have the survival advantage. Therefore, this results in the formation of a new species of green beetles, which are reproductively isolated.

c. ​Fossils are the remains of organisms that once existed on Earth. They represent the ancestors of plants and animals, which are alive even today. Fossils provide evidences of evolution by revealing the characteristics of the past organisms, and the changes that have occurred in these organisms to give rise to a present organism. Fossils have the same shape as that of the original animal, but their colour and texture may vary widely. The colour of a fossil depends upon the type of minerals that form it. For example, the fossil of a bone will not have some constituents of the bone in it. It has the same shape as the bone, but it is chemically more like a rock.​
Importance of Fossils:
(i) They inform us about the types of living things that existed in the past.
(ii) They inform us about the extent to which living things have changed over time.
(iii) The most recent fossil is found in a rock nearest to earth’s surface. Therefore, they inform us about the time when a particular life form existed.

d. Fetal science or embryology is used as an evidence of evolution. Comparative study of embryos in vertebrates/chordates shows that there is lot of similarity in them at the initial stages whereas this similarity decreases gradually. This similarity in the development of embryos represents common origin of organisms.