The Fact of Evolution
The Earth started out as a hot, lifeless ball of molten rock. The first rocks were formed when the planet cooled, but even then the Earth looked nothing like it does today. Water filled the ocean basins, but no fish swarm in it and no algae floated on it. All of the billions of different life forms that would someday develop were absent in this early stage.
The transition from a lifeless planet to one that teems with living things came in two stages. The first stage involved the appearance of the first living cell from the lifeless chemical compounds that existed in the early Earth, and was governed by the laws of chemistry and physics. The second stage was the multiplication, diversification, and evolution of that first living cell into astonishing variety of life that we observed on Earth now.
Today,—virtually all scientists accept evolution as a historical fact— an observational phenomenon as well established at the fact that the Earth goes around the Sun or that a ball falls when you drop it.
This acceptance of evolution as a fact does not mean that there are not any debates about different theories of evolution—how fast it proceeds and by what mechanisms. However, differences in opinion about these details should not be confused with the fact that life has evolved on Earth for billions of years. A tremendous body of scientific literature is developed to this subject, however, I will go over the three important pieces of evidence for the process of evolution¦ the fossil record, the evidence of biochemical similarity and the occurrence of vestigial organs.
The Fossil Record:
When a plant or animal dies, the remains are usually lost. A tree will rot, the carcass of an animal will be torn apart by scavengers and dispersed, a crab shell will be broken up by the action of the surf. Occasionally, however, an organism is removed quickly from the environment, typically by being buried in sediments and sealed off. The hard parts of such an organism may remain underground for a long period of time.
As times goes by, two things may happen. First of all, the material around the organism may go through the rock cycle and turned into rock. Second, materials in the water flowing through the surrounding area may gradually replace the calcium and other atoms in the buried hard parts, thus creating a fossil. We often think of fossils as the large dinosaur skeletons in a museum, but the term is also used to refer to other kinds of records of past life, such as a leaf imprint on mud that changes into rock, or an insect preserved in amber.
The term fossil record refers to all of the fossils that are known to exists. They are cataloged and analyzed since human beings first began to study them in a systematic way in the early part of the nineteenth century. The fossil record reveals how different organisms came to be what they are. The fossil record of horses, for example, include a series of precursor animals beginning with one about the size of a cat some 50 million years ago, and changing through many intermediate forms up to modern times. Throughout this sequence of fossil mammals is a gradual transition from a small quick animal to a large grazing one.
The fossil record also contains some examples of actual changes in species. In order to do this, the fossil record had to be very complete, with many thousands of years of continuous sediments. Such continuity is rare, but in some instances the transitions from one species to another has documentation.
Even so, the major problem with the fossil record is that it is very incomplete. It is estimated that only one species (not one individual) out of every 10,000 early life forms is represented in the fossil record. Thus, in interpreting the past, we must always be aware that we are dealing with a very small and select sample of what actually there. This sample is strongly biased toward organisms that were more likely to have been buried soon after death. We have a much better record. of mollusks and clams that lived on the continental shelf, therefore, than we do of insects that flew around primeval forests. Nevertheless, the fossil record was the first (and for a long time, the only) evidence that backed up the notion that life constantly changing and evolving.
STOP AND THINK!!
From the preceding discussion, animals with skeletons and shells will be fossilized more easily than those with soft bodies. What animals alive today are likely to be found as fossils a million years from know? Would future paleontologists get an accurate view of present–day life by examining those fossils.
Three key ideas quickly emerged from studies of fossils. First, the older the rocks, the more their animal and plant fossils differ from modern forms. Mammals in 5–million–year–old rocks are not terribly different from today's fauna, but few species that existed 50 million years ago would be recognizable today, and dinosaurs rather than mammals were dominate 150 million years ago. Similar patterns occur in shells, plants fish, and all other forms. Often the earlier forms appear to combine characteristics of later organisms. Ancient insects preserved in amber, for example, show some forms that may be intermediate between ants and wasps. Early mammals, similarly, have general mammalian characteristics, but few of the specialized structures that have evolved in bats, whales, tigers, and rabbits.
Fossils also display general trends in overall complexity of form. All known fossils from before about 570 million years ago are either single–celled organisms or simple invertebrates such as jellyfish. Marine invertebrates with hard parts — mollusks, coral and crustacea, for example—dominate the record for the next 200 million years. Simple land animals and plants appear next, followed by flowering plants and a much greater variety of large land animals. This long–term trend toward increasing complexity of organization is consistent with all theories of evolution.
Finally, the fossil record proves beyond a doubt that most species lived on Earth have died out and our now extinct. Scientists estimate that for every species on the planet today, as many as 999 species have become extinct at some point in the past. In fact, the average lifetime of a species in the fossil record seems to be a few million years. Species, like individuals, are born live out their life, and die. This fact alone is ample evidence that some natural mechanism must exist to produce new species as the old disappear.
