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In the beginning, there was...
OVER the decades, there has been considerable controversy over the evolution of man and also about the age of Earth, the only planet in the solar system which supports life as we know it.
Fossils — the remains of animals and plants — collectively represent the evidence of life on Earth through time. This short study attempts to provide a means to determine the distribution of life in the geologic past.
There are two groups of animals: vertebrates (animals with backbones); and, invertebrates (animals without them). Consequently, two kinds of animal fossils are found in ancient rocks.
The history of our planet has been divided into several eras on the basis of the life-forms existing in each of them. These eras, in their ascending order, are the Archeozoic, Proterozoic, Paleozoic, Mesozoic and Cenozoic —ranging in age from 5,000 million years to the present.
As a whole, the eras cover the time it took Mother Nature to create Earth and the life on it. The part of history dating back to 600 million years or more is called Precambrian, which began with the formation of the planet and includes the Archeozoic and Proterozoic eras.
It stretches back to about five million years ago — the time when the Earth was created. Only a few single-celled organisms remain as evidence of this age.
The Paleozoic era, which followed the Precambrian, comprises the Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian periods. This era began about 600 million years ago and lasted for about 370 million years.
Mesozoic, which followed the Paleozoic era, includes Triassic, Jurassic and Crete periods and ranges from 230 to 70 million years ago. After the Mesozoic era came the Cenozoic, which began about 70 million years ago and lasted for about 65 million years. This era encompasses the tertiary and quaternary periods.
Since its formation, Earth has witnessed life both in the seas and on land, in the forms of animals and plants. During most of the eras, the seas covered much of the land and were full of life-forms, which were scanty on land.
The research carried out on the beginnings of life indicates that single-celled organisms, which existed millions of years ago, came first. Scientists, with the help of powerful microscopes have found the existence of fossilized bacteria and green algae in about 3- to 3.5-billion-year-old rocks, which cover the Archeozoic and Protozoic eras of the geological time scale.
However, common remains of ancient life normally date back to Paleozoic and succeeding eras. In the early Paleozoic era (about 570 million years ago) a great abundance of diverse plants and animals, such as clams, sponges and corals, lived in the oceans. Trilobites — primitive arthropods (invertebrates with segmented body and jointed limbs) — were mainly present in America and Europe but were not common in the other parts of the world.
These became extinct in the Permian period, about 250 million years ago, and no longer exist. The scientists call this era “The Age of Trilobites.” With trilobites other vertebrates, such as brachiopods, ammonites, bivalves, gastropods, echinoderms and belemnites, were common in the Cambrian, Ordovician, and Silurian periods (570 to 431 million years ago).
A large animal of the early Paleozoic era (Ordovician period) was cephalopod which had a 3-metre-long shell. Trace fossils of burrows, tracks or trails are indirect evidence of ancient life, particularly those of the invertebrates like corals and clams which date back to the early Paleozoic era.
The seas covered much of the land until the middle of the Paleozoic era (about 400 million years ago) and marine life was abundant. Simple plants, with undeveloped separate stems and leaves, appeared during the Ordovician-Silurian period (485 to 410 million years ago). The first woody plant, however, appeared about 400 million years ago.
Vertebrates — such as fish and amphibians (creatures like fogs, toads, and newts, living both in the water and on land), reptiles, birds and mammals (vertebrates that feed milk to their young ones) — appeared on Earth millions of years later. The first vertebrate life, in the form of fish, appeared in the early Ordovician period (about 500 million years ago). But it was in the Devonian period (410 to 345 million years ago) that the fish really arrived and their fossil record became representative in nature. The Devonian period is often called “The Age of Fish.”
The first land life was amphibian, which was traced back to the late Devonian period with the help of remains found in Greenland in 345-million-year-old rocks. The amphibians were the dominant creatures on land and they did not face much challenge until the arrival on the scene of the dinosaurs. This happened in the Mesozoic era (230 to 70 million years ago).
Coal, the fossilized remains of trees and plants, can be traced back to the Carboniferous period (350 to 280 million years ago). The first reptilian life also lived during this period. The last period of the Paleozoic era is Permian in which reptilian life of normal size was common. Trilobites became extinct during this period.
After the Paleozoic era came the Mesozoic era, which began about 230 million years ago and lasted for about 180 million years. During this era not only was the invertebrate life common in the seas but also on land. The dinosaurs, the largest living vertebrate, was the dominant force of the time. They were present on all the continents of the world during the entire Mesozoic era. Therefore, scientists call this era “The Age of Dinosaurs.”
It is generally thought that although the first mammal had made its appearance in the early parts of this era, it could not make much progress due to the presence of dinosaurs. In Triassic and Jurassic periods —the earliest periods of the Mesozoic era — invertebrates such as ammonites were many. Lizard-hipped dinosaurs also became common.
In the Jurassic period, birds and mammals evolved. Bird-hipped dinosaurs were common in Cretaceous period, the last part of the era. Among invertebrates, ammonites, belemnites and oysters were abundant.
As long as dinosaurs were dominant, the other land animals did not flourish. It was in the late Cretaceous period (about 70 million years ago) that dinosaurs disappeared from the face of Earth. The disappearance of dinosaurs paved the way for the mammalian life to grow.
The part of history that followed was the Cenozoic era. This era started about 65 million years ago and consists of Tertiary and Quaternary periods. These periods are further divided into Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene and Holocene epochs.
During this era, the mammals flourished and became a highly varied life-form, after getting rid of the world’s largest creature, the dinosaur, who died out at the end of Cretaceous period. Grasses evolved during the Cenozoic era and ultimately became an important food for the mammals.
In the Paleocene and Eocene epochs — the earliest parts of the Cenozoic era — the mammalian life developed on land. The Eocene period, which began about 55 million years ago and lasted for about 15 million years, is specially significant for mammals because it marks the presence of several modern groups such as rodents, cetaceans, ungulates and birds such as eagles, pelicans, quails, and vultures. This era is known as “The Age of Mammals.”
Marine life — the corals, clams and snail — was dominant in the seas. The mammalian life, with vertebrates and invertebrates, is continuing both in the seas and on land all over the world today.
The period ranging from 40 to 2 million years ago covers Oligocene, Miocene and Pliocene epochs of the Cenozoic era. In this period, the mammals became common and varied on land. About 25 million years ago most of the animals in the world were similar to the modern ones.
Sites in Pakistan
In this regard, Pakistan is not different from the rest of the world. It is among the regions which have rocks that contain the fossilized remains of ancient life-forms. Sedimentary rocks, both of the marine and continental types, are widely exposed in the four provinces of Pakistan and Azad Kashmir. This is the reason why, over the years, the areas which now form Pakistan have attracted many palaeontologists. The scientists have particularly been interested in the Salt Range and in North-West Frontier Province.
It is a matter of great scientific significance for the country that of the old and young life-forms which have been discovered so far in the world, many have also been detected here. Evidence of marine life — including lingula, redlicha, hyoliths, rushtonia, sprifer and digging trilobites — have all been found in the Salt Range region. These date back to the early Paleozoic era (600 to 400 million year ago).
Similarly, marine life-forms, including brachiopods, bivalves, gastropods, cephalopods, ammo-noids, corals and belemnites have been detected in the country, which are between 280 and 24 million years old. Continental life-forms — in the shape of 70- to 2-million-year-old reptiles and mammals — have also been discovered, especially in parts of Balochistan and Potohar.
The remains of the oldest dinosaurs and mammals to be found in Pakistan — 70 and 55 million years old respectively — were discovered in the Barkhan and Sor Range areas of Balochistan. Evidence of a 52 million years old whale was discovered both in the Punjab and NWFP. In addition, not long ago the world’s largest extinct Rhinoceros, named the “Balochi-therium,” was found in 22-million-year-old rocks in the Sul area of Balochistan.
Mammalian life in the form of the primitive artiodactyl — an even-toed animal known as “Diacodexis” — has been found in Banda Daud Shah, NWFP, in about 52-million-year-old rocks. A rock unit known as Mancher Formation, exposed in the Gaj Nat and Sehwan Sharif areas of Dadu District, contains the remains of a continental life which are about 15 million years old. Reportedly the youngest fossilized jaw of a deer, which is 1.8 to 2 million years old, came from areas near Rewat, which is about 14 kilometres south of Rawalpindi.
In Pakistan, the remains of ancient life are at display at the Pakistan Museum of Natural History, Islamabad, and the Geological Survey of Pakistan Museum, Quetta. Both these museums should be improved upon.
The writer is a former deputy director of the Geological Survey of Pakistan
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