Today, there are millions of species on Earth. If we include the species that have become extinct in geological history, the number would exceed hundreds of millions. They come in various shapes and sizes, with some having simple structures and others extremely complex ones. They were not created by any supernatural deity, but evolved step by step from the simplest prokaryotic organisms. Evidence of biological evolution can be found in fossils.
About 3.5 billion years ago, the simplest and most primitive prokaryotic cells (single-celled organisms composed of a cell membrane, cell wall, and cytoplasm) appeared on Earth—the cyanobacteria (also known as blue-green algae). By 2.5 billion years ago, only fossils of prokaryotic cells had been found in Earth's strata. If the fossils of eukaryotic organisms (organisms with a cell membrane, cytoplasm, various organelles, and a nucleus) found in 20-billion-year-old strata are reliable, the emergence of eukaryotes should have occurred no later than 20 billion years ago. Subsequently, the earliest animal fossils evolved from algae. The earliest animals and plants were not very different. For example, the earliest green algae, which contained chlorophyll and could photosynthesize, were called algae by some scholars, while others called them flagellates because they could move with flagella.
In subsequent strata, people found fossils of worms, followed by fossils of hemichordates or chordates, fish fossils belonging to vertebrates (such as the early Cambrian fish fossils in Kunming, China), amphibian fossils, and reptile fossils. Finally, they found mammal (including human) and bird fossils. At each stage of biological evolution, many fossil evidences can be found. Therefore, fossils provide a unique historical clue to Darwin's theory of evolution, and together they help scientists draw a "tree of life" of geological history. Although this tree of life is not complete, without fossils, we would not be able to understand when and how various past and present organisms evolved step by step, nor would we know who their ancestors were.
More importantly, fossils also provide clues and evidence for many missing links in biological evolution, such as feathered dinosaur fossils, the fossil of Archaeopteryx, three-toed horse fossils, and numerous ancient human fossils. Through them, we seem to be able to witness the miraculous process of biological evolution with our own eyes. Therefore, since Darwin proposed the theory of evolution, fossils have always been the most powerful and direct evidence to support this theory.