Evidence for a Common Ancestor of Organisms

Section 1: Evidence for a Common Ancestor of Organisms

Darwin's theory of biological evolution consists mainly of two doctrines: the doctrine of common descent and the doctrine of natural selection. The former posits that all organisms on Earth evolved from a common primitive ancestor, while the latter reveals the mechanisms of biological evolution and explains the formation of adaptations and species.

Darwin cited a wealth of evidence to support his theory. Later, as research progressed, many new pieces of evidence supporting the theory of evolution were discovered.

Evidence from Fossils Found in Strata

Fossils refer to the remains, traces, or impressions of ancient organisms preserved in the Earth's strata through natural processes.

Fossils can be used to determine the types, morphology, structure, behavior, and other characteristics of organisms that once lived on Earth. For example, the diet of animals can be inferred from fossilized teeth, the size and locomotion of animals can be deduced from skeletal fossils, and the morphology, structure, and taxonomic position of plants can be inferred from plant fossils (Figure 6-1). Therefore, fossils are the most direct and important evidence for studying biological evolution.

Most fossils are found in sedimentary rock layers (Figure 6-2). If the strata are compared to a book, fossils are like the text within, recording the history of the Earth and the evolution of life.

A large amount of fossil evidence discovered so far confirms that organisms gradually evolved from a common primitive ancestor over long geological periods. This evidence also reveals the sequence of evolution, from simple to complex organisms, from lower to higher forms, and from aquatic to terrestrial life.

Traces of Evolution in Modern Organisms — Other Evidence

The evolution of life has left traces not only in the strata but also in modern organisms, which serve as evidence supporting the theory of evolution.

Comparative Anatomy Evidence
By studying the morphology and structure of the organs and systems of vertebrates, comparative anatomy can provide evidence for whether these organisms share a common ancestor. Observe the wings of a bat, the fins of a whale, the forelimbs of a cat, and the arms of a human in Figure 6-3. Can you identify any common features among them?

Embryological Evidence
Embryology is the study of the formation and development of embryos in plants and animals. By comparing the embryonic development of different animals and humans, clues about evolution can be found. For example, in the early stages of human embryonic development, gill slits and a tail appear (Figure 6-4), which are very similar to the early embryonic development of fish. However, as development progresses, the gill slits and tail disappear in humans, while adult fish retain their gills and tails. Moreover, vertebrates share similar stages in early embryonic development, providing evidence that humans and other vertebrates have a common ancestor.

Evidence at the Cellular and Molecular Levels
From tiny yeast cells to vast banyan trees, from climbing chimpanzees to humans, there are significant differences between various organisms. However, research in cell biology and molecular biology shows that seemingly distinct modern organisms also share many similarities.

At the cellular and molecular levels, modern organisms exhibit many common features. For example, they all have cells capable of metabolism, growth, and reproduction, and these cells share common material and structural foundations. This strongly supports the idea that organisms share a common ancestor. The similarities in DNA, proteins, and other biological macromolecules among different organisms suggest that modern organisms have a common primitive ancestor. The degree of these similarities indicates the closeness of the evolutionary relationships among different species and the order in which they appeared in evolutionary history.

In conclusion, fossils provide direct evidence for studying biological evolution, while comparative anatomy, embryology, and research at the cellular and molecular levels offer strong support for the theory of evolution. These pieces of evidence complement and corroborate each other, strongly supporting Darwin's doctrine of common descent, and providing a solid foundation for explaining adaptation and the formation of species.