Humans have attempted to understand and interpret fossils since the days of the ancient Greeks and Romans. More scientific views of fossils began to emerge during the Renaissance, but fossils were still largely misunderstood and misinterpreted.
When Darwin wrote the Origin of Species, the oldest animal fossils known were those from the Cambrian Period, about 540 million years old. The scarcity and relatively young age of fossils and especially the rarity of intermediate fossil forms held implications that worried Darwin concerning the validity of his theories.
Since Darwin’s time, the fossil record has been pushed back to 3.5 billion years before the present and many of the gaps in the fossil record have been filled in by the research of paleontologists.
What is a Fossil?
A fossil (from Latin, fossus, having been dug up) is any remains, impressions, or traces of living organisms from past geological ages that have been preserved in the earth’s crust. Paleontological studies of fossils across geologic time and how they formed are vital to our understanding of the evolutionary relationships (phylogeny) between taxa (taxonomic groups). Large specimens (macrofossils) are more commonly found and displayed, although tiny remains (microfossils) are actually far more prevalent in the fossil record.
How Do Fossils Form?
For fossilization to take place, the traces and remains of organisms must be quickly buried so that weathering and decomposition do not occur. Skeletal structures or other hard parts of organisms such as shells are the most commonly occurring form of fossilized remains.
Evidence of past life may also be recorded as molds and casts, such as the footprints of a dinosaur, and as imprints, such as the shadowy outlines of a leaf that originally fell into soft sediments and left an impression.
As an organism dies, the organic materials gradually decay, such that the bones or bark and wood become porous. If the creature is subsequently buried in mud, minerals will infiltrate into the decaying organic materials and gradually fill up the pores. These mineralized structures will harden into stones through a process known as petrifaction (think petrified wood) and be preserved as fossils.
The dead bodies of organisms may also be well preserved in ice, in hardened resin of coniferous trees (amber), in tar, or in anaerobic, acidic peat.
The Extent of the Fossil Record
To hold or even see a fossil is to transcend millions of years of time to a prehistoric world we can barely imagine. Despite the relative rarity of suitable conditions for fossilization, approximately 250,000 fossil species are known. The number of individual fossils this represents varies greatly from species to species, but many millions of fossils have been recovered: for instance, more than three million fossils from the last Ice Age have been recovered from the La Brea Tar Pits in Los Angeles
Interpreting the Fossil Record
The fossil record is evidence to support the contention that all life forms on the planet are derived from ancient common ancestors. While the fossils cannot undoubtedly prove common descent, they are highly suggestive of it if they show two patterns: older forms are simpler than newer forms and the number of species increases with time.
The fossil record certainly meets the first criterion. For example, among the earliest mammalian fossils, there are no specialized mammals like whales, but we do find fossils of whale-like terrestrial mammals that possessed underdeveloped legs. The second criterion poses a sort of impasse between evolutionary scientists who claim their findings to be incomplete yet compelling and creationists who bemoan the entire fossil record as severely lacking.
Limitations of the Fossil Record
The fossil record is an important source for scientists when tracing the evolutionary history of organisms. However, because of limitations inherent in the record, there are not fine scales of intermediate forms between related groups of species. This lack of continuous fossils in the record is a major limitation in tracing the descent of biological groups. Furthermore, there are also much larger gaps between major evolutionary lineages. When transitional fossils are found that show intermediate forms in what had previously been a gap in knowledge, they are often popularly referred to as "missing links".
Evidence of Evolution
Fossils one of several lines of evidence that reveal the ongoing and dynamic nature of the process of evolution. Other pieces of evidence are biogeography, comparative anatomy, and the molecular record.
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