Fossils, from Latin fossus, literally having been dug up are the mineralized or otherwise preserved remains or traces such as footprints of animals, plants, and other organisms. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous or fossil-containing rock formations and sedimentary layers or strata is known as the fossil record. The study of fossils across geological time, how they were formed, and the evolutionary relationships between taxa or phylogeny are some of the most important functions of the science of paleontology. Using radiometric dating techniques, geologists have determined most fossils to be several thousands to several billions of years old. Yet there is no minimum age for a fossil. Fossils vary in size from microscopic, such as single cells, to gigantic, such as dinosaurs. A fossil normally preserves only a portion of the deceased organism, usually that portion that was partially mineralized during life, such as the bones and teeth of vertebrates, or the chitinous exoskeletons of invertebrates. Preservation of soft tissues is exquisitely rare in the fossil record. Fossils may also consist of the marks left behind by the organism while it was alive, such as the footprint or faeces of a reptile. These types of fossil are called trace fossils or ichnofossils as opposed to body fossils. Finally, past life leaves some markers that cannot be seen but can be detected in the form of biochemical signals; these are known as chemical fossils or biomarkers.
Places of exceptional fossilization. Fossil sites with exceptional preservation, sometimes including preserved soft tissues are known as Lagerstätten. These formations may have resulted from carcass burial in an anoxic environment with minimal bacteria, thus delaying decomposition. Lagerstätten span geological time from the Cambrian period to the present. Worldwide, some of the best examples of near-perfect fossilization are the Cambrian Maotianshan shales and Burgess Shale, the Devonian Hunsrück Slates, the Jurassic Solnhofen limestone, and the Carboniferous Mazon Creek localities. Paleobiologists have studied the frozen flesh of prehistoric mammoths, and the preserved muscle tissue of archosaurs.

Earliest fossiliferous sites. Earth’s oldest fossils are the stromatolites consisting of rock built from layer upon layer of sediment and precipitants. Based on studies of now-rare but living stromatolites specifically, certain blue-green bacteria, the growth of fossil stromatolitic structures was biogenetically mediated by mats of microorganisms through their entrapment of sediments. However, abiotic mechanisms for stromatolitic growth are also known, leading to a decades-long and sometimes-contentious scientific debate regarding biogenesis of certain formations, especially those from the lower to middle Archaean eon.
It is more widely accepted that stromatolites from the late Archaean and through the middle Proterozoic eon were mostly formed by massive colonies of cyanobacteria formerly known as blue-green algae, and that the oxygen byproduct of their photosynthetic metabolism first resulted in earth’s massive banded iron formations and subsequently oxygenated earth’s atmosphere. Though rare, microstructures resembling cells are sometimes found within stromatolites; but these are also the source of scientific contention. The Gunflint Chert contains abundant microfossils widely accepted as a diverse consortium of 2.0 bya microbes. In contrast, putative fossil cyanobacteria cells from the 3.4 bya Warrawoona Group in Western Australia are in dispute since abiotic processes cannot be ruled out. Confirmation of the Warrawoona microstructures as cyanobacteria would profoundly impact our understanding of when and how early life diversified, pushing important evolutionary milestones further back in time. The continued study of these oldest fossils is paramount to calibrate complementary molecular phylogenetics models.
Developments in interpretation of the fossil record. Ever since recorded history began, and probably before, people have found fossils, pieces of rock and minerals which have replaced the remains of biologic organisms or preserved their external form. These fossils, and the totality of their occurrence within the sequence of Earth’s rock strata is referred to as the fossil record.
The fossil record was one of the early sources of data relevant to the study of evolution and continues to be relevant to the history of life on Earth. Paleontologists examine the fossil record in order to understand the process of evolution and the way particular species have evolved. Various explanations have been put forth throughout history to explain what fossils are and how they came to be where they were found. Many of these explanations relied on folktales or mythologies. In China the fossil bones of ancient mammals including Homo erectus were often mistaken for dragon bones and used as medicine and aphrodisiacs. In the West the presence of fossilized sea creatures high up on mountainsides was seen as proof of the biblical deluge. More scientific views of fossils began to emerge during the Renaissance. For example, Leonardo Da Vinci noticed discrepancies with the use of the biblical flood narrative as an explanation for fossil origins:
“If the Deluge had carried the shells for distances of three and four hundred miles from the sea it would have carried them mixed with various other natural objects all heaped up together; but even at such distances from the sea we see the oysters all together and also the shellfish and the cuttlefish and all the other shells which congregate together, found all together dead; and the solitary shells are found apart from one another as we see them every day on the sea-shores.
And we find oysters together in very large families, among which some may be seen with their shells still joined together, indicating that they were left there by the sea and that they were still living when the strait of Gibraltar was cut through. In the mountains of Parma and Piacenza multitudes of shells and corals with holes may be seen still sticking to the rocks…”
William Smith (1769-1839), an English canal engineer, observed that rocks of different ages based on the law of superposition preserved different assemblages of fossils, and that these assemblages succeeded one another in a regular and determinable order. He observed that rocks from distant locations could be correlated based on the fossils they contained. He termed this the principle of faunal succession. Smith, who preceded Charles Darwin, was unaware of biological evolution and did not know why faunal succession occurred. Biological evolution explains why faunal succession exists: as different organisms evolve, change and go extinct, they leave behind fossils. Faunal succession was one of the chief pieces of evidence cited by Darwin that biological evolution had occurred. Early naturalists well understood the similarities and differences of living species leading Linnaeus to develop a hierarchical classification system still in use today. It was Darwin and his contemporaries who first linked the hierarchical structure of the great tree of life in living organisms with the then very sparse fossil record. Darwin eloquently described a process of descent with modification, or evolution, whereby organisms either adapt to natural and changing environmental pressures, or they perish. When Charles Darwin wrote On the Origin of Species, the oldest animal fossils were those from the Cambrian Period, now known to be about 540 million years old. The absence of older fossils worried Darwin about the implications for the validity of his theories, but he expressed hope that such fossils would be found, noting that only a small portion of the world is known with accuracy. Darwin also pondered the sudden appearance of many groups in the oldest known Cambrian fossiliferous strata. Since Darwin’s time, the fossil record has been pushed back to 3.5 billion years before the present. Most of these Precambrian fossils are microscopic bacteria or microfossils. However, macroscopic fossils are now known from the late Proterozoic. The Ediacaran biota also called Vendian biota dating from 575 million years ago collectively constitutes a richly diverse assembly of early multicellular eukaryotes. The fossil record and faunal succession form the basis of the science of biostratigraphy or determining the age of rocks based on the fossils they contain. For the first 150 years of geology, biostratigraphy and superposition were the only means for determining the relative age of rocks. The geologic time scale was developed based on the relative ages of rock strata as determined by the early paleontologists and stratigraphers. Since the early years of the twentieth century, absolute dating methods, such as radiometric dating including potassium/argon, argon/argon, uranium series, and carbon-14 dating have been used to verify the relative ages obtained by fossils and to provide absolute ages for many fossils. Radiometric dating has shown that the earliest known fossils are over 3.5 billion years old. Various dating methods have been used and are used today depending on local geology and context, and while there is some variance in the results from these dating methods, nearly all of them provide evidence for a very old Earth, approximately 4.6 billion years. “The fossil record is life’s evolutionary epic that unfolded over four billion years as environmental conditions and genetic potential interacted in accordance with natural selection.” The earth’s climate, tectonics, atmosphere, oceans, and periodic disasters invoked the primary selective pressures on all organisms, which they either adapted to, or they perished with or without leaving descendants. Modern paleontology has joined with evolutionary biology to share the interdisciplinary task of unfolding the tree of life, which inevitably leads backwards in time to the microscopic life of the Precambrian when cell structure and functions evolved. Earth’s deep time in the Proterozoic and deeper still in the Archaean is only “recounted by microscopic fossils and subtle chemical signals”. Molecular biologists, using phylogenetics, can compare protein amino acid or nucleotide sequence homology to infer taxonomy and evolutionary distances among organisms, but with limited statistical confidence. The study of fossils, on the other hand, can more specifically pin point when and in what organism branching occurred in the tree of life. Modern phylogenetics and paleontology work together in the clarification of science’s still dim view of the appearance life and its evolution during deep time on earth. Niles Eldredge’s study of the Phacops trilobite genus supported the hypothesis that modifications to the arrangement of the trilobite’s eye lenses proceeded by fits and starts over millions of years during the Devonian. Eldredge’s interpretation of the Phacops fossil record was that the aftermaths of the lens changes, but not the rapidly occurring evolutionary process, were fossilised. This and other data led Stephen Jay Gould and Niles Eldredge to publish the seminal paper on punctuated equilibrium in 1971. An example of modern paleontological progress is the application of synchrotron X-ray tomographic techniques to early Cambrian bilaterian embryonic microfossils that has recently yielded new insights of metazoan evolution at its earliest stages. The tomography technique provides previously unattainable three-dimensional resolution at the limits of fossilization. Fossils of two enigmatic bilaterians, the worm-like Markuelia and a putative, primitive protostome, Pseudooides, provide a peek at germ layer embryonic development. These 543 Ma old embryos support the emergence of some aspects of arthropod development earlier than previously thought in the late Proterozoic. The preserved embryos from China and Siberia underwent rapid diagenetic phosphatization resulting in exquisite preservation, including cell structures. This research is a notable example of how knowledge encoded by the fossil record continues to contribute otherwise unattainable information on the emergence and development of life on Earth. For example, the research suggests Markuelia has closest affinity to priapulid worms, and is adjacent to the evolutionary branching of Priapulida, Nematoda and Arthropoda.
Even with the wealth of information now known about fossils, some groups maintain non-scientific beliefs based on the earlier views of the fossil record.