Fossils provide a record of life on Earth from as long as 1 billion years ago, showing how organisms, animals and plants have changed over time. These records can be dated, so they are a physical representation of evolution, such as the way that ancient fish developed a bone in their jaw over time.
According to BioLogos, fossils originating from about 1 billion years ago show that the earliest life on the planet consisted of organisms that were single-celled. The fossils from about half a billion years later show that those single cells had developed into organisms with multiple cells. In other words, the fossils show that organisms evolved over a long period of time, moving from simpler life to more complex life. This pattern continued throughout history, as life forms became more and more complex.
Any single fossil is largely useless as evidence of evolution, but over time scientists have discovered a significant fossil record of various species. These fossils are often clearly recognizable as ancestors of modern species. Fossils are also consistent within geological regions, showing that these gradually developing species mostly lived in the same area and allowing scientists to further study isolated populations. Incremental changes are also thoroughly documented for some species. For example, the evolutionary record of the change from the prehistoric Eohippus to the modern horse is well-documented; Eohippus was a short, four-toed equid that lived 60 million years ago and gradually developed into the tall, single-toed modern equine.
Evolution does not go in a single line, evolving from one form of life to the next. In fact, according to the Smithsonian website, homo sapiens evolved in a large family of other human species, although homo sapiens (the species of humans on the planet now) were the only species to survive.
Although there are some gaps and other unexplained anomalies in the fossil record, the bulk of the evidence strongly supports the theory of evolution. Other theories that have been advanced, such as creationism, strongly contradict the fossil record. Scientists continue to study and learn about fossils in the hope of explaining any anomalies and gaining a more thorough understanding of the evolutionary process, and as of 2014, scientific evidence supports evolution.