Eradication of infectious diseases

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Eradication_of_infectious_diseases 

 

A child suffering from smallpox. In 1980, the World Health Organization announced the global eradication of smallpox. It is the only human disease to be eradicated worldwide.

 

Play media

Video recording of a set of presentations given in 2010 about humanity's efforts towards malaria eradication

Eradication is the reduction of an infectious disease's prevalence in the global host population to zero. It is sometimes confused with elimination, which describes either the reduction of an infectious disease's prevalence in a regional population to zero or the reduction of the global prevalence to a negligible amount. Further confusion arises from the use of the term eradication to refer to the total removal of a given pathogen from an individual (also known as clearance of an infection), particularly in the context of HIV and certain other viruses where such cures are sought.

The selection of infectious diseases for eradication is based on rigorous criteria, as both biological and technical features determine whether a pathogenic organism is (at least potentially) eradicable. The targeted organism must not have a non-human reservoir (or, in the case of animal diseases, the infection reservoir must be an easily identifiable species, as in the case of rinderpest), and/or amplify in the environment. This implies that sufficient information on the life cycle and transmission dynamics is available at the time an eradication initiative is programmed. An efficient and practical intervention (such as a vaccine or antibiotic) must be available to interrupt transmission of the infective agent. Studies of measles in the pre-vaccination era led to the concept of the critical community size, the size of the population below which a pathogen ceases to circulate. The use of vaccination programs before the introduction of an eradication campaign can reduce the susceptible population. The disease to be eradicated should be clearly identifiable, and an accurate diagnostic tool should exist. Economic considerations, as well as societal and political support and commitment, are other crucial factors that determine eradication feasibility.

Two infectious diseases have successfully been eradicated: smallpox and rinderpest. There are also four ongoing programs, targeting poliomyelitis, yaws, dracunculiasis, and malaria. Five more infectious diseases have been identified as of April 2008 as potentially eradicable with current technology by the Carter Center International Task Force for Disease Eradication—measles, mumps, rubella, lymphatic filariasis and cysticercosis.

Eradicated diseases

So far, two diseases have been successfully eradicated—one specifically affecting humans (smallpox), and one affecting a wide range of ruminants (rinderpest).

Smallpox

Boy with smallpox (1969). The last natural smallpox case was of Ali Maow Maalin, in Merca, Somalia, on 26 October 1977.

Smallpox was the first disease, and so far the only infectious disease of humans, to be eradicated by deliberate intervention. It became the first disease for which there was an effective vaccine in 1798 when Edward Jenner showed the protective effect of inoculation (vaccination) of humans with material from cowpox lesions.

Smallpox (variola) occurred in two clinical varieties: variola major, with a mortality rate of up to 40 percent, and variola minor, also known as alastrim, with a mortality rate of less than one percent. The last naturally occurring case of Variola major was diagnosed in October 1975 in Bangladesh. The last naturally occurring case of smallpox (Variola minor) was diagnosed on 26 October 1977, on Ali Maow Maalin, in the Merca District, of Somalia. The source of this case was a known outbreak in the nearby district of Kurtuware. All 211 contacts were traced, revaccinated, and kept under surveillance.

After two years' detailed analysis of national records, the global eradication of smallpox was certified by an international commission of smallpox clinicians and medical scientists on 9 December 1979, and endorsed by the General Assembly of the World Health Organization on 8 May 1980. However, there is an ongoing debate regarding the continued storage of the smallpox virus by labs in the US and Russia, as any accidental or deliberate release could create a new epidemic in people born since the late 1980s due to the cessation of vaccinations against the smallpox virus.

Rinderpest

During the twentieth century, there were a series of campaigns to eradicate rinderpest, a viral disease which infected cattle and other ruminants and belonged to the same family as measles, primarily through the use of a live attenuated vaccine. The final, successful campaign was led by the Food and Agriculture Organization of the United Nations. On 14 October 2010, with no diagnoses for nine years, the Food and Agriculture Organization announced that the disease had been completely eradicated, making this the first (and so far the only) disease of livestock to have been eradicated by human undertakings.

Global eradication underway

Poliomyelitis (polio)

International wild poliovirus cases by year

Year	Estimated	Recorded
1975	—	49,293
1980	400,000	52,552
1985	—	38,637
1988	350,000	35,251
1990	—	23,484
1993	100,000	10,487
1995	—	7,035
2000	—	2,971
2005	—	1,998
2010	—	1,352
2011	—	650
2012	—	222
2013	—	385
2014	—	359
2015	—	74
2016	—	37
2017	—	22
2018	—	33
2019	—	176

A dramatic reduction of the incidence of poliomyelitis in industrialized countries followed the development of a vaccine in the 1950s. In 1960, Czechoslovakia became the first country certified to have eliminated polio.

In 1988, the World Health Organization (WHO), Rotary International, the United Nations Children's Fund (UNICEF), and the United States Centers for Disease Control and Prevention (CDC) passed the Global Polio Eradication Initiative. Its goal was to eradicate polio by the year 2000. The updated strategic plan for 2004–2008 expects to achieve global eradication by interrupting poliovirus transmission, using the strategies of routine immunization, supplementary immunization campaigns, and surveillance of possible outbreaks. The WHO estimates that global savings from eradication, due to forgone treatment and disability costs, could exceed one billion U.S. dollars per year.

The following world regions have been declared polio-free:

• The Americas (1994)
• Indo-West Pacific region (1997)
• Europe (1998)
• Western Pacific region, including China (2000)
• Southeast Asia region (2014), including India
• Africa (2020)

The lowest annual wild polio prevalence seen so far was in 2017, with only 22 reported cases, although there were more total reported cases (including circulated vaccine-derived cases) than in 2016, mainly due to reporting of circulated vaccine-derived cases in Syria, where it likely had already been circulating, but gone unreported, presumably due to the civil war. Only two or three countries remain in which poliovirus transmission may never have been interrupted: Pakistan, Afghanistan, and perhaps Nigeria. (There have been no cases caused by wild strains of poliovirus in Nigeria since August 2016, though cVDPV2 was detected in environmental samples in 2017.) Nigeria was removed from the WHO list of polio-endemic countries in September 2015 but added back in 2016, and India was removed in 2014 after no new cases were reported for one year.

On 20 September 2015, the World Health Organization announced that wild poliovirus type 2 had been eradicated worldwide, as it has not been seen since 1999. On 24 October 2019, the World Health Organization announced that wild poliovirus type 3 had also been eradicated worldwide. This leaves only wild poliovirus type 1 and circulating vaccine-derived polio circulating in a few isolated pockets, with all wild polio cases after August 2016 in Afghanistan and Pakistan.

Dracunculiasis

International Guinea worm cases by year

Year	Reported cases	Countries
1989	892,055	16
1995	129,852	19
2000	75,223	16
2005	10,674	12
2010	1,797	6
2011	1,060	4
2012	542	4
2013	148	5
2014	126	4
2015	22	4
2016	25	3
2017	30	2
2018	28	3
2019	54	4
2020	24	4

Dracunculiasis, also called Guinea worm disease, is a painful and disabling parasitic disease caused by the nematode Dracunculus medinensis. It is spread through consumption of drinking water infested with copepods hosting Dracunculus larvae. The Carter Center has led the effort to eradicate the disease, along with the CDC, the WHO, UNICEF, and the Bill and Melinda Gates Foundation.

Unlike diseases such as smallpox and polio, there is no vaccine or drug therapy for guinea worm. Eradication efforts have been based on making drinking water supplies safer (e.g. by provision of borehole wells, or through treating the water with larvicide), on containment of infection and on education for safe drinking water practices. These strategies have produced many successes: two decades of eradication efforts have reduced Guinea worm's global incidence to 22 cases in 2015, after which cases rose to 25 cases in 2016, and 30 cases in 2017, but this is still down from an estimated 3.5 million in 1986. Success has been slower than was hoped—the original goal for eradication was 1995. The WHO has certified 180 countries free of the disease, and only three countries—South Sudan, Ethiopia, and Chad—reported cases of guinea worm in 2016, and only two—Ethiopia and Chad—in 2017. As of 2010, the WHO predicted it would be "a few years yet" before eradication is achieved, on the basis that it took 6–12 years for the countries that have so far eliminated guinea worm transmission to do so after reporting a similar number of cases to that reported by Sudan in 2009. The number of cases in 2019 (54) was less than 2% of the number in 2009, so real progress has been made towards this prediction. Nonetheless, the last 1% may be the hardest, and cases have increased from 2015 (22) to 2019 (54). The worm is able to infect dogs, domestic cats and baboons as well as humans, complicating eradication efforts.

Yaws

Yaws is a rarely fatal but highly disfiguring disease caused by the spiral-shaped bacterium (spirochete) Treponema pallidum pertenue, a close relative of the syphilis bacterium Treponema pallidum pallidum, spread through skin to skin contact with infectious lesions. The global prevalence of this disease and the other endemic treponematoses, bejel and pinta, was reduced by the Global Control of Treponematoses (TCP) programme between 1952 and 1964 from about 50 million cases to about 2.5 million (a 95% reduction). However, following the cessation of this program these diseases remained at a low prevalence in parts of Asia, Africa and the Americas with sporadic outbreaks. According to a 2012 official WHO roadmap, the elimination should be achievable by 2020. Yaws is currently targeted by the South-East Asian Regional Office of the WHO for elimination from the remaining endemic countries in this region (India, Indonesia and East Timor) by 2010, and so far, this appears to have met with some success, since no cases have been seen in India since 2004. The discovery that oral antibiotic azithromycin can be used instead of the previous standard, injected penicillin, was tested on Lihir Island from 2013 to 2014; a single oral dose of the macrolide antibiotic reduced disease prevalence from 2.4% to 0.3% at 12 months. The campaign was in an early stage in 2013, still gathering data on disease incidence and planning initial large-scale treatment campaigns in Cameroon, Ghana, Indonesia, Papua New Guinea, the Solomon Islands, and Vanuatu.

Malaria

1962 Pakistani postage stamp promoting malaria eradication program

Malaria has been eliminated from most of Europe, North America, Australia, North Africa and the Caribbean, and parts of South America, Asia and Southern Africa. The WHO defines elimination as having no domestic transmission for the past three years. They also define an "elimination stage" when a country is on the verge of eliminating malaria, as being less than one case per 1000 people at risk per year. As of 2019, 38 countries are certified as having eliminated malaria. As of 2018, 21 countries were seeking to eliminate malaria by 2020. The pre-elimination stage entails fewer than 5 cases per 1000 people at risk per year.

In 1955 the WHO launched the Global Malaria Eradication Program (GMEP). Support waned, and the program was suspended in 1969. Since 2000, support for eradication has increased, although some people in the global health community remain sceptical. According to the WHO's World Malaria Report 2015, the global mortality rate for malaria fell by 60% between 2000 and 2015. The WHO aims to achieve a further 90% reduction between 2015 and 2030. Bill Gates believes that global eradication is possible by 2040.

A major challenge to malaria elimination is the persistence of malaria in border regions, making international cooperation crucial.

Regional elimination established or underway

Some diseases have already been eliminated from large regions of the world, and/or are currently being targeted for regional elimination. This is sometimes described as "eradication", although technically the term only applies when this is achieved on a global scale. Even after regional elimination is successful, interventions often need to continue to prevent a disease becoming re-established. Three of the diseases here listed (lymphatic filariasis, measles, and rubella) are among the diseases believed to be potentially eradicable by the International Task Force for Disease Eradication, and if successful, regional elimination programs may yet prove a stepping stone to later global eradication programs. This section does not cover elimination where it is used to mean control programs sufficiently tight to reduce the burden of an infectious disease or other health problem to a level where they may be deemed to have little impact on public health, such as the leprosy, neonatal tetanus, or obstetric fistula campaigns.

Hookworm

In North American countries, such as the United States, elimination of hookworm had been attained due to scientific advances. Despite the United States declaring that it had eliminated hookworm decades ago, a 2017 study showed it was present in Lowndes County, Alabama.

The Rockefeller Foundation's hookworm campaign in the 1920s was supposed to focus on the eradication of hookworm infections for those living in Mexico and other rural areas. However, the campaign was politically influenced, causing it to be less successful, and regions such as Mexico still deal with these infections from parasitic worms. This use of health campaigns by political leaders for political and economic advantages has been termed the science-politics paradox.

Lymphatic filariasis

Lymphatic filariasis is an infection of the lymph system by mosquito-borne microfilarial worms which can cause elephantiasis. Studies have demonstrated that transmission of the infection can be broken when a single dose of combined oral medicines is consistently maintained annually for approximately seven years. The strategy for eliminating transmission of lymphatic filariasis is mass distribution of medicines that kill the microfilariae and stop transmission of the parasite by mosquitoes in endemic communities. In sub-Saharan Africa, albendazole is being used with ivermectin to treat the disease, whereas elsewhere in the world albendazole is used with diethylcarbamazine. Using a combination of treatments better reduces the number of microfilariae in blood. Avoiding mosquito bites, such as by using insecticide-treated mosquito bed nets, also reduces the transmission of lymphatic filariasis. In the Americas, 95% of the burden of lymphatic filariasis is on the island of Hispaniola (comprising Haiti and the Dominican Republic). An elimination effort to address this is currently under way alongside the malaria effort described above; both countries intend to eliminate the disease by 2020.

As of October 2008, the efforts of the Global Programme to Eliminate LF are estimated to have already prevented 6.6 million new filariasis cases from developing in children, and to have stopped the progression of the disease in another 9.5 million people who have already contracted it. Overall, of 83 endemic countries, mass treatment has been rolled out in 48, and elimination of transmission reportedly achieved in 21.

Measles

Five out of six WHO regions have goals to eliminate measles, and at the 63rd World Health Assembly in May 2010, delegates agreed to move towards eventual eradication, although no specific global target date has yet been agreed. The Americas set a goal in 1994 to eliminate measles and rubella transmission by 2000, and successfully achieved regional measles elimination in 2002, although there have been occasional small outbreaks from imported cases since then. Europe had set a goal to eliminate measles transmission by 2010, but were hindered by the MMR vaccine controversy and by low uptake in certain groups, and despite achieving low levels by 2008, European countries have since experienced a small resurgence in cases. They have set a new target of 2015. The Eastern Mediterranean also had goals to eliminate measles by 2010 (later revised to 2015), the Western Pacific aims to eliminate the disease by 2012, and in 2009 the regional committee for Africa agreed a goal of measles elimination by 2020. As of May 2010, only the South-East Asian region has yet to set a target date for elimination of measles transmission.

In 2005, a global target was agreed for a 90% reduction in measles deaths by 2010 from the 757,000 deaths in 2000; estimates for 2008 show a 78% decline so far to 164,000 deaths. However, some have been pushing to attempt global eradication. This was updated at the 2010 World Health Assembly to a targeted 95% reduction in mortality by 2015, alongside specific vaccination and structural targets, and in a meeting in November 2010, the Strategic Advisory Group of Experts on Immunization "concluded that measles can and should be eradicated". A study of the costs of eradicating measles compared to the costs of maintaining indefinite control was commissioned in 2009 by the WHO and the Bill and Melinda Gates Foundation. In 2013, measles deaths globally were down to 145,700.

As of mid-2013, measles elimination in many areas is stalling. "This year, measles and rubella outbreaks are occurring in many areas of the world where people have no immunity to these viruses. The reasons people are unvaccinated range from lack of access to vaccines in areas of insecurity, to poor performing health systems, to vaccine refusals. We need to address each of these challenges if we’re going to meet global measles and rubella elimination goals," said Dr. Myrna Charles of the American Red Cross, as reported in a post in the Measles and Rubella Initiative's blog. A look at the WHO's epidemiological graph of measles over time from 2008-2013 show that, with little more of two years to go to 2015, measles cases in 2013 are moving in the wrong direction, with more cases this year than at the same point in 2012 or 2011.

During 2014 there were 23 outbreaks of measles in the United States and over 600 individual cases, which is the highest seen in decades. In 2015 the US has had one major outbreak of measles originating from an amusement park in California of a variant of the virus circulating in the Philippines in 2014. From this there have been 113 individual measles cases and one death (out of the total of 189 cases in the US in 2015).

The WHO region of the Americas declared on 27 September 2016 it had eliminated measles. The last confirmed endemic case of measles in the Americas was in Brazil in July 2015. May 2017 saw a return of measles to the US after an outbreak in Minnesota among unvaccinated children. Another outbreak occurred in the state of New York between 2018 and 2019, causing over 200 confirmed measles cases in mostly ultra-Orthodox Jewish communities. Subsequent outbreaks occurred in New Jersey and Washington state with over 30 cases reported in the Pacific Northwest.

Rubella

Four out of six WHO regions have goals to eliminate rubella, with the WHO recommending using existing measles programmes for vaccination with combined vaccines such as the MMR vaccine. The number of reported cases dropped from 670 thousand in the year 2000 to below 15 thousand in 2018, and the global coverage of rubella vaccination was estimated at 69% in 2018 by the WHO. The WHO region of the Americas declared on 29 April 2015 it had eliminated rubella and congenital rubella syndrome. The last confirmed endemic case of rubella in the Americas was in Argentina in February 2009. Australia achieved eradication in 2018. The WHO European region missed its elimination target of 2010 due to undervaccination in Central and Western Europe; it has set a new goal of 2015. The disease remains problematic in other regions; the WHO regions of Africa and South-East Asia have the highest rates of congenital rubella syndrome and a 2013 outbreak of rubella in Japan resulted in 15,000 cases.

Onchocerciasis

Onchocerciasis (river blindness) is the world's second leading cause of infectious blindness. It is caused by the nematode Onchocerca volvulus, which is transmitted to people via the bite of a black fly. Elimination of this disease is under way in the region of the Americas, where this disease was endemic to Brazil, Colombia, Ecuador, Guatemala, Mexico and Venezuela. The principal tool being used is mass ivermectin treatment. If successful, the only remaining endemic locations would be in Africa and Yemen. In Africa, it is estimated that greater than 102 million people in 19 countries are at high risk of onchocerciasis infection, and in 2008, 56.7 million people in 15 of these countries received community-directed treatment with ivermectin. Since adopting such treatment measures in 1997, the African Programme for Onchocerciasis Control reports a reduction in the prevalence of onchocerciasis in the countries under its mandate from a pre-intervention level of 46.5% in 1995 to 28.5% in 2008. Some African countries, such as Uganda, are also attempting elimination and successful elimination was reported in 2009 from two endemic foci in Mali and Senegal.

On 29 July 2013, the Pan American Health Organization (PAHO) announced that after 16 years of efforts, Colombia had become the first country in the world to eliminate the parasitic disease onchocerciasis. It has also been eliminated in Ecuador (2014), Mexico (2015), and Guatemala (2016).

Bovine spongiform encephalopathy (BSE) and new variant Creutzfeldt–Jakob disease (vCJD)

Following an epidemic of variant Creutzfeldt–Jakob disease (vCJD) in the UK in the 1990s, there have been campaigns to eliminate bovine spongiform encephalopathy (BSE) in cattle across the European Union and beyond which have achieved large reductions in the number of cattle with this disease. Cases of vCJD have also fallen since then, from an annual peak of 29 cases in 2000 to five in 2008 and none in 2012. Two cases were reported in both 2013 and 2014: two in France; one in the United Kingdom and one in the United States.

Following the ongoing eradication effort, only seven cases of BSE were reported worldwide in 2013: three in the United Kingdom, two in France, one in Ireland and one in Poland. This is the lowest number of cases since at least 1988. In 2015 there were at least six reported cases (three of the atypical H-type.

Syphilis

In 2015, Cuba became the first country in the world to eliminate mother-to-child syphilis. In 2017 the WHO declared that Antigua and Barbuda, Saint Kitts and Nevis and four British Overseas Territories—Anguilla, Bermuda, Cayman Islands, and Montserrat—have been certified that they have ended transmission of mother-to-child syphilis and HIV. Nevertheless eradication of syphilis by all transmission methods remains unresolved and many questions about the eradication effort remain to be answered.

African trypanosomiasis

Early planning by the WHO for the eradication of African trypanosomiasis, also known as sleeping sickness, is underway as the rate of reported cases continues to decline and passive treatment is continued. The WHO aims to completely eliminate transmission of the Trypanosoma brucei gambiense parasite by 2030, though it acknowledges that this goal "leaves no room for complacency."

Rabies

Rabies-free countries and territories as of 2018

Because the rabies virus is almost always caught from animals, rabies eradication has focused on reducing the population of wild and stray animals, controls and compulsory quarantine on animals entering the country, and vaccination of pets and wild animals. Many island nations, including Iceland, Ireland, Japan, Malta, and the United Kingdom, managed to eliminate rabies during the twentieth century, and more recently much of continental Europe has been declared rabies-free.

Eradicable diseases in animals

As far as animal diseases are concerned, now that rinderpest has been stamped out, many experts believe peste des petits ruminants (PPR) is the next disease amenable to global eradication. Also known as goat plague or ovine rinderpest, PPR is a highly contagious viral disease of goats and sheep characterized by fever, painful sores in the mouth, tongue and feet, diarrhea, pneumonia and death, especially in young animals. It is caused by a virus of the genus Morbillivirus that is related to rinderpest, measles and canine distemper.

Eradication difficulties

Public upheaval by means of war, famine, political means, and infrastructure destruction can disrupt or eliminate eradication efforts altogether.

Fossil

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Fossil

A fossil (from Classical Latin: fossilis, literally "obtained by digging") is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in amber, hair, petrified wood, oil, coal, and DNA remnants. The totality of fossils is known as the fossil record.

Paleontology is the study of fossils: their age, method of formation, and evolutionary significance. Specimens are usually considered to be fossils if they are over 10,000 years old. The oldest fossils are around 3.48 billion years old to 4.1 billion years old. The observation in the 19th century that certain fossils were associated with certain rock strata led to the recognition of a geological timescale and the relative ages of different fossils. The development of radiometric dating techniques in the early 20th century allowed scientists to quantitatively measure the absolute ages of rocks and the fossils they host.

There are many processes that lead to fossilization, including permineralization, casts and molds, authigenic mineralization, replacement and recrystallization, adpression, carbonization, and bioimmuration.

Fossils vary in size from one-micrometre (1 µm) bacteria to dinosaurs and trees, many meters long and weighing many tons. 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 or calcareous exoskeletons of invertebrates. Fossils may also consist of the marks left behind by the organism while it was alive, such as animal tracks or feces (coprolites). These types of fossil are called trace fossils or ichnofossils, as opposed to body fossils. Some fossils are biochemical and are called chemofossils or biosignatures.

Fossilization processes

The process of fossilization varies according to tissue type and external conditions.

Permineralization

Permineralized bryozoan from the Devonian of Wisconsin.

Permineralization is a process of fossilization that occurs when an organism is buried. The empty spaces within an organism (spaces filled with liquid or gas during life) become filled with mineral-rich groundwater. Minerals precipitate from the groundwater, occupying the empty spaces. This process can occur in very small spaces, such as within the cell wall of a plant cell. Small scale permineralization can produce very detailed fossils. For permineralization to occur, the organism must become covered by sediment soon after death, otherwise the remains are destroyed by scavengers or decomposition. The degree to which the remains are decayed when covered determines the later details of the fossil. Some fossils consist only of skeletal remains or teeth; other fossils contain traces of skin, feathers or even soft tissues. This is a form of diagenesis.

Casts and molds

External mold of a bivalve from the Logan Formation, Lower Carboniferous, Ohio

In some cases, the original remains of the organism completely dissolve or are otherwise destroyed. The remaining organism-shaped hole in the rock is called an external mold. If this hole is later filled with other minerals, it is a cast. An endocast, or internal mold, is formed when sediments or minerals fill the internal cavity of an organism, such as the inside of a bivalve or snail or the hollow of a skull.

Authigenic mineralization

This is a special form of cast and mold formation. If the chemistry is right, the organism (or fragment of organism) can act as a nucleus for the precipitation of minerals such as siderite, resulting in a nodule forming around it. If this happens rapidly before significant decay to the organic tissue, very fine three-dimensional morphological detail can be preserved. Nodules from the Carboniferous Mazon Creek fossil beds of Illinois, USA, are among the best documented examples of such mineralization.

Replacement and recrystallization

Silicified (replaced with silica) fossils from the Road Canyon Formation (Middle Permian of Texas)

 

Recrystallized scleractinian coral (aragonite to calcite) from the Jurassic of southern Israel

Replacement occurs when the shell, bone, or other tissue is replaced with another mineral. In some cases mineral replacement of the original shell occurs so gradually and at such fine scales that microstructural features are preserved despite the total loss of original material. A shell is said to be recrystallized when the original skeletal compounds are still present but in a different crystal form, as from aragonite to calcite.

Adpression (compression-impression)

Compression fossils, such as those of fossil ferns, are the result of chemical reduction of the complex organic molecules composing the organism's tissues. In this case the fossil consists of original material, albeit in a geochemically altered state. This chemical change is an expression of diagenesis. Often what remains is a carbonaceous film known as a phytoleim, in which case the fossil is known as a compression. Often, however, the phytoleim is lost and all that remains is an impression of the organism in the rock—an impression fossil. In many cases, however, compressions and impressions occur together. For instance, when the rock is broken open, the phytoleim will often be attached to one part (compression), whereas the counterpart will just be an impression. For this reason, one term covers the two modes of preservation: adpression.

Soft tissue, cell and molecular preservation

Because of their antiquity, an unexpected exception to the alteration of an organism's tissues by chemical reduction of the complex organic molecules during fossilization has been the discovery of soft tissue in dinosaur fossils, including blood vessels, and the isolation of proteins and evidence for DNA fragments. In 2014, Mary Schweitzer and her colleagues reported the presence of iron particles (goethite-aFeO(OH)) associated with soft tissues recovered from dinosaur fossils. Based on various experiments that studied the interaction of iron in haemoglobin with blood vessel tissue they proposed that solution hypoxia coupled with iron chelation enhances the stability and preservation of soft tissue and provides the basis for an explanation for the unforeseen preservation of fossil soft tissues. However, a slightly older study based on eight taxa ranging in time from the Devonian to the Jurassic found that reasonably well-preserved fibrils that probably represent collagen were preserved in all these fossils and that the quality of preservation depended mostly on the arrangement of the collagen fibers, with tight packing favoring good preservation. There seemed to be no correlation between geological age and quality of preservation, within that timeframe.

Carbonization and coalification

Fossils that are carbonized or coalified consist of the organic remains which have been reduced primarily to the chemical element carbon. Carbonized fossils consist of a thin film which forms a silhouette of the original organism, and the original organic remains were typically soft tissues. Coalified fossils consist primarily of coal, and the original organic remains were typically woody in composition.

• 

  Carbonized fossil of a possible leech from the Silurian Waukesha Biota of Wisconsin.

• 

  Partially coalified axis (branch) of a lycopod from the Devonian of Wisconsin.

Bioimmuration

The star-shaped holes (Catellocaula vallata) in this Upper Ordovician bryozoan represent a soft-bodied organism preserved by bioimmuration in the bryozoan skeleton.

Bioimmuration occurs when a skeletal organism overgrows or otherwise subsumes another organism, preserving the latter, or an impression of it, within the skeleton. Usually it is a sessile skeletal organism, such as a bryozoan or an oyster, which grows along a substrate, covering other sessile sclerobionts. Sometimes the bioimmured organism is soft-bodied and is then preserved in negative relief as a kind of external mold. There are also cases where an organism settles on top of a living skeletal organism that grows upwards, preserving the settler in its skeleton. Bioimmuration is known in the fossil record from the Ordovician to the Recent.

Types

Examples of index fossils

Index

Index fossils (also known as guide fossils, indicator fossils or zone fossils) are fossils used to define and identify geologic periods (or faunal stages). They work on the premise that, although different sediments may look different depending on the conditions under which they were deposited, they may include the remains of the same species of fossil. The shorter the species' time range, the more precisely different sediments can be correlated, and so rapidly evolving species' fossils are particularly valuable. The best index fossils are common, easy to identify at species level and have a broad distribution—otherwise the likelihood of finding and recognizing one in the two sediments is poor.

Trace

Trace fossils consist mainly of tracks and burrows, but also include coprolites (fossil feces) and marks left by feeding. Trace fossils are particularly significant because they represent a data source that is not limited to animals with easily fossilized hard parts, and they reflect animal behaviours. Many traces date from significantly earlier than the body fossils of animals that are thought to have been capable of making them. Whilst exact assignment of trace fossils to their makers is generally impossible, traces may for example provide the earliest physical evidence of the appearance of moderately complex animals (comparable to earthworms).

Coprolites are classified as trace fossils as opposed to body fossils, as they give evidence for the animal's behaviour (in this case, diet) rather than morphology. They were first described by William Buckland in 1829. Prior to this they were known as "fossil fir cones" and "bezoar stones." They serve a valuable purpose in paleontology because they provide direct evidence of the predation and diet of extinct organisms. Coprolites may range in size from a few millimetres to over 60 centimetres.

• 

  Cambrian trace fossils including Rusophycus, made by a trilobite

• 

  A coprolite of a carnivorous dinosaur found in southwestern Saskatchewan

• 

  Densely packed, subaerial or nearshore trackways (Climactichnites wilsoni) made by a putative, slug-like mollusk on a Cambrian tidal flat

Transitional

A transitional fossil is any fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group. This is especially important where the descendant group is sharply differentiated by gross anatomy and mode of living from the ancestral group. Because of the incompleteness of the fossil record, there is usually no way to know exactly how close a transitional fossil is to the point of divergence. These fossils serve as a reminder that taxonomic divisions are human constructs that have been imposed in hindsight on a continuum of variation.

Microfossils

Microfossils about 1 mm

Microfossil is a descriptive term applied to fossilized plants and animals whose size is just at or below the level at which the fossil can be analyzed by the naked eye. A commonly applied cutoff point between "micro" and "macro" fossils is 1 mm. Microfossils may either be complete (or near-complete) organisms in themselves (such as the marine plankters foraminifera and coccolithophores) or component parts (such as small teeth or spores) of larger animals or plants. Microfossils are of critical importance as a reservoir of paleoclimate information, and are also commonly used by biostratigraphers to assist in the correlation of rock units.

Resin

The wasp Leptofoenus pittfieldae trapped in Dominican amber, from 20 to 16 million years ago. It is known only from this specimen.

Fossil resin (colloquially called amber) is a natural polymer found in many types of strata throughout the world, even the Arctic. The oldest fossil resin dates to the Triassic, though most dates to the Cenozoic. The excretion of the resin by certain plants is thought to be an evolutionary adaptation for protection from insects and to seal wounds. Fossil resin often contains other fossils called inclusions that were captured by the sticky resin. These include bacteria, fungi, other plants, and animals. Animal inclusions are usually small invertebrates, predominantly arthropods such as insects and spiders, and only extremely rarely a vertebrate such as a small lizard. Preservation of inclusions can be exquisite, including small fragments of DNA.

Derived, or reworked

Eroded Jurassic plesiosaur vertebral centrum found in the Lower Cretaceous Faringdon Sponge Gravels in Faringdon, England. An example of a remanié fossil.

A derived, reworked or remanié fossil is a fossil found in rock that accumulated significantly later than when the fossilized animal or plant died. Reworked fossils are created by erosion exhuming (freeing) fossils from the rock formation in which they were originally deposited and their redeposition in a younger sedimentary deposit.

Wood

Petrified wood. The internal structure of the tree and bark are maintained in the permineralization process.

 

Polished section of petrified wood showing annual rings

Fossil wood is wood that is preserved in the fossil record. Wood is usually the part of a plant that is best preserved (and most easily found). Fossil wood may or may not be petrified. The fossil wood may be the only part of the plant that has been preserved: therefore such wood may get a special kind of botanical name. This will usually include "xylon" and a term indicating its presumed affinity, such as Araucarioxylon (wood of Araucaria or some related genus), Palmoxylon (wood of an indeterminate palm), or Castanoxylon (wood of an indeterminate chinkapin).

Subfossil

The term subfossil can be used to refer to remains, such as bones, nests, or defecations, whose fossilization process is not complete, either because the length of time since the animal involved was living is too short (less than 10,000 years) or because the conditions in which the remains were buried were not optimal for fossilization. Subfossils are often found in caves or other shelters where they can be preserved for thousands of years. The main importance of subfossil vs. fossil remains is that the former contain organic material, which can be used for radiocarbon dating or extraction and sequencing of DNA, protein, or other biomolecules. Additionally, isotope ratios can provide much information about the ecological conditions under which extinct animals lived. Subfossils are useful for studying the evolutionary history of an environment and can be important to studies in paleoclimatology.

Subfossils are often found in depositionary environments, such as lake sediments, oceanic sediments, and soils. Once deposited, physical and chemical weathering can alter the state of preservation.

Chemical fossils

Chemical fossils, or chemofossils, are chemicals found in rocks and fossil fuels (petroleum, coal, and natural gas) that provide an organic signature for ancient life. Molecular fossils and isotope ratios represent two types of chemical fossils. The oldest traces of life on Earth are fossils of this type, including carbon isotope anomalies found in zircons that imply the existence of life as early as 4.1 billion years ago.

Dating

Estimating dates

Paleontology seeks to map out how life evolved across geologic time. A substantial hurdle is the difficulty of working out fossil ages. Beds that preserve fossils typically lack the radioactive elements needed for radiometric dating. This technique is our only means of giving rocks greater than about 50 million years old an absolute age, and can be accurate to within 0.5% or better. Although radiometric dating requires careful laboratory work, its basic principle is simple: the rates at which various radioactive elements decay are known, and so the ratio of the radioactive element to its decay products shows how long ago the radioactive element was incorporated into the rock. Radioactive elements are common only in rocks with a volcanic origin, and so the only fossil-bearing rocks that can be dated radiometrically are volcanic ash layers, which may provide termini for the intervening sediments.

Stratigraphy

Consequently, palaeontologists rely on stratigraphy to date fossils. Stratigraphy is the science of deciphering the "layer-cake" that is the sedimentary record. Rocks normally form relatively horizontal layers, with each layer younger than the one underneath it. If a fossil is found between two layers whose ages are known, the fossil's age is claimed to lie between the two known ages. Because rock sequences are not continuous, but may be broken up by faults or periods of erosion, it is very difficult to match up rock beds that are not directly adjacent. However, fossils of species that survived for a relatively short time can be used to match isolated rocks: this technique is called biostratigraphy. For instance, the conodont Eoplacognathus pseudoplanus has a short range in the Middle Ordovician period. If rocks of unknown age have traces of E. pseudoplanus, they have a mid-Ordovician age. Such index fossils must be distinctive, be globally distributed and occupy a short time range to be useful. Misleading results are produced if the index fossils are incorrectly dated. Stratigraphy and biostratigraphy can in general provide only relative dating (A was before B), which is often sufficient for studying evolution. However, this is difficult for some time periods, because of the problems involved in matching rocks of the same age across continents. Family-tree relationships also help to narrow down the date when lineages first appeared. For instance, if fossils of B or C date to X million years ago and the calculated "family tree" says A was an ancestor of B and C, then A must have evolved earlier.

It is also possible to estimate how long ago two living clades diverged, in other words approximately how long ago their last common ancestor must have lived, by assuming that DNA mutations accumulate at a constant rate. These "molecular clocks", however, are fallible, and provide only approximate timing: for example, they are not sufficiently precise and reliable for estimating when the groups that feature in the Cambrian explosion first evolved, and estimates produced by different techniques may vary by a factor of two.

Limitations

Some of the most remarkable gaps in the fossil record (as of October 2013) show slanting toward organisms with hard parts.

Organisms are only rarely preserved as fossils in the best of circumstances, and only a fraction of such fossils have been discovered. This is illustrated by the fact that the number of species known through the fossil record is less than 5% of the number of known living species, suggesting that the number of species known through fossils must be far less than 1% of all the species that have ever lived. Because of the specialized and rare circumstances required for a biological structure to fossilize, only a small percentage of life-forms can be expected to be represented in discoveries, and each discovery represents only a snapshot of the process of evolution. The transition itself can only be illustrated and corroborated by transitional fossils, which will never demonstrate an exact half-way point.

The fossil record is strongly biased toward organisms with hard-parts, leaving most groups of soft-bodied organisms with little to no role. It is replete with the mollusks, the vertebrates, the echinoderms, the brachiopods and some groups of arthropods.

Sites

Lagerstätten

Fossil sites with exceptional preservation—sometimes including preserved soft tissues—are known as Lagerstätten—German for "storage places". These formations may have resulted from carcass burial in an anoxic environment with minimal bacteria, thus slowing 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.

Stromatolites

Lower Proterozoic stromatolites from Bolivia, South America

Stromatolites are layered accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria. Stromatolites provide some of the most ancient fossil records of life on Earth, dating back more than 3.5 billion years ago.

Stromatolites were much more abundant in Precambrian times. While older, Archean fossil remains are presumed to be colonies of cyanobacteria, younger (that is, Proterozoic) fossils may be primordial forms of the eukaryote chlorophytes (that is, green algae). One genus of stromatolite very common in the geologic record is Collenia. The earliest stromatolite of confirmed microbial origin dates to 2.724 billion years ago.

A 2009 discovery provides strong evidence of microbial stromatolites extending as far back as 3.45 billion years ago.

Stromatolites are a major constituent of the fossil record for life's first 3.5 billion years, peaking about 1.25 billion years ago. They subsequently declined in abundance and diversity, which by the start of the Cambrian had fallen to 20% of their peak. The most widely supported explanation is that stromatolite builders fell victims to grazing creatures (the Cambrian substrate revolution), implying that sufficiently complex organisms were common over 1 billion years ago.

The connection between grazer and stromatolite abundance is well documented in the younger Ordovician evolutionary radiation; stromatolite abundance also increased after the end-Ordovician and end-Permian extinctions decimated marine animals, falling back to earlier levels as marine animals recovered. Fluctuations in metazoan population and diversity may not have been the only factor in the reduction in stromatolite abundance. Factors such as the chemistry of the environment may have been responsible for changes.

While prokaryotic cyanobacteria themselves reproduce asexually through cell division, they were instrumental in priming the environment for the evolutionary development of more complex eukaryotic organisms. Cyanobacteria (as well as extremophile Gammaproteobacteria) are thought to be largely responsible for increasing the amount of oxygen in the primeval earth's atmosphere through their continuing photosynthesis. Cyanobacteria use water, carbon dioxide and sunlight to create their food. A layer of mucus often forms over mats of cyanobacterial cells. In modern microbial mats, debris from the surrounding habitat can become trapped within the mucus, which can be cemented by the calcium carbonate to grow thin laminations of limestone. These laminations can accrete over time, resulting in the banded pattern common to stromatolites. The domal morphology of biological stromatolites is the result of the vertical growth necessary for the continued infiltration of sunlight to the organisms for photosynthesis. Layered spherical growth structures termed oncolites are similar to stromatolites and are also known from the fossil record. Thrombolites are poorly laminated or non-laminated clotted structures formed by cyanobacteria common in the fossil record and in modern sediments.

The Zebra River Canyon area of the Kubis platform in the deeply dissected Zaris Mountains of southwestern Namibia provides an extremely well exposed example of the thrombolite-stromatolite-metazoan reefs that developed during the Proterozoic period, the stromatolites here being better developed in updip locations under conditions of higher current velocities and greater sediment influx.

Astrobiology

It has been suggested that biominerals could be important indicators of extraterrestrial life and thus could play an important role in the search for past or present life on the planet Mars. Furthermore, organic components (biosignatures) that are often associated with biominerals are believed to play crucial roles in both pre-biotic and biotic reactions.

On 24 January 2014, NASA reported that current studies by the Curiosity and Opportunity rovers on Mars will now be searching for evidence of ancient life, including a biosphere based on autotrophic, chemotrophic and/or chemolithoautotrophic microorganisms, as well as ancient water, including fluvio-lacustrine environments (plains related to ancient rivers or lakes) that may have been habitable. The search for evidence of habitability, taphonomy (related to fossils), and organic carbon on the planet Mars is now a primary NASA objective.

Pseudofossils

An example of a pseudofossil: Manganese dendrites on a limestone bedding plane from Solnhofen, Germany; scale in mm

Pseudofossils are visual patterns in rocks that are produced by geologic processes rather than biologic processes. They can easily be mistaken for real fossils. Some pseudofossils, such as geological dendrite crystals, are formed by naturally occurring fissures in the rock that get filled up by percolating minerals. Other types of pseudofossils are kidney ore (round shapes in iron ore) and moss agates, which look like moss or plant leaves. Concretions, spherical or ovoid-shaped nodules found in some sedimentary strata, were once thought to be dinosaur eggs, and are often mistaken for fossils as well.

History of the study of fossils

Gathering fossils dates at least to the beginning of recorded history. The fossils themselves are referred to as the fossil record. The fossil record was one of the early sources of data underlying the study of evolution and continues to be relevant to the history of life on Earth. Paleontologists examine the fossil record to understand the process of evolution and the way particular species have evolved.

Ancient civilizations

Fossils have been visible and common throughout most of natural history, and so documented human interaction with them goes back as far as recorded history, or earlier.

There are many examples of paleolithic stone knives in Europe, with fossil echinoderms set precisely at the hand grip, going all the way back to Homo heidelbergensis and neanderthals. These ancient peoples also drilled holes through the center of those round fossil shells, apparently using them as beads for necklaces.

The ancient Egyptians gathered fossils of species that resembled the bones of modern species they worshipped. The god Set was associated with the hippopotamus, therefore fossilized bones of hippo-like species were kept in that deity's temples. Five-rayed fossil sea urchin shells were associated with the deity Sopdu, the Morning Star, equivalent of Venus in Roman mythology.

Ceratopsian skulls are common in the Dzungarian Gate mountain pass in Asia, an area once famous for gold mines, as well as its endlessly cold winds. This has been attributed to legends of both gryphons and the land of Hyperborea

Fossils appear to have directly contributed to the mythology of many civilizations, including the ancient Greeks. Classical Greek historian Herodotos wrote of an area near Hyperborea where gryphons protected golden treasure. There was indeed gold mining in that approximate region, where beaked Protoceratops skulls were common as fossils.

A later Greek scholar, Aristotle, eventually realized that fossil seashells from rocks were similar to those found on the beach, indicating the fossils were once living animals. He had previously explained them in terms of vaporous exhalations, which Persian polymath Avicenna modified into the theory of petrifying fluids (succus lapidificatus). Recognition of fossil seashells as originating in the sea was built upon in the 14th century by Albert of Saxony, and accepted in some form by most naturalists by the 16th century.

Roman naturalist Pliny the Elder wrote of "tongue stones", which he called glossopetra. These were fossil shark teeth, thought by some classical cultures to look like the tongues of people or snakes. He also wrote about the horns of Ammon, which are fossil ammonites, from whence the group of shelled octopus-cousins ultimately draws its modern name. Pliny also makes one of the earlier known references to toadstones, thought until the 18th century to be a magical cure for poison originating in the heads of toads, but which are fossil teeth from Lepidotes, a Cretaceous ray-finned fish.

The Plains tribes of North America are thought to have similarly associated fossils, such as the many intact pterosaur fossils naturally exposed in the region, with their own mythology of the thunderbird.

There is no such direct mythological connection known from prehistoric Africa, but there is considerable evidence of tribes there excavating and moving fossils to ceremonial sites, apparently treating them with some reverence.

In Japan, fossil shark teeth were associated with the mythical tengu, thought to be the razor-sharp claws of the creature, documented some time after the 8th century AD.

In medieval China, the fossil bones of ancient mammals including Homo erectus were often mistaken for "dragon bones" and used as medicine and aphrodisiacs. In addition, some of these fossil bones are collected as "art" by scholars, who left scripts on various artifacts, indicating the time they were added to a collection. One good example is the famous scholar Huang Tingjian of the South Song Dynasty during the 11th century, who kept a specific seashell fossil with his own poem engraved on it. In the West fossilized sea creatures on mountainsides were seen as proof of the biblical deluge.

In 1027, the Persian Avicenna explained fossils' stoniness in The Book of Healing:

If what is said concerning the petrifaction of animals and plants is true, the cause of this (phenomenon) is a powerful mineralizing and petrifying virtue which arises in certain stony spots, or emanates suddenly from the earth during earthquake and subsidences, and petrifies whatever comes into contact with it. As a matter of fact, the petrifaction of the bodies of plants and animals is not more extraordinary than the transformation of waters.

From the 13th century to the present day, scholars pointed out that the fossil skulls of Deinotherium giganteum, found in Crete and Greece, might have been interpreted as being the skulls of the Cyclopes of Greek mythology, and are possibly the origin of that Greek myth. Their skulls appear to have a single eye-hole in the front, just like their modern elephant cousins, though in fact it's actually the opening for their trunk.

Fossil shells from the cretaceous era sea urchin, Micraster, were used in medieval times as both shepherd's crowns to protect houses, and as painted fairy loaves by bakers to bring luck to their bread-making.

In Norse mythology, echinoderm shells (the round five-part button left over from a sea urchin) were associated with the god Thor, not only being incorporated in thunderstones, representations of Thor's hammer and subsequent hammer-shaped crosses as Christianity was adopted, but also kept in houses to garner Thor's protection.

These grew into the shepherd's crowns of English folklore, used for decoration and as good luck charms, placed by the doorway of homes and churches. In Suffolk, a different species was used as a good-luck charm by bakers, who referred to them as fairy loaves, associating them with the similarly shaped loaves of bread they baked.

Early modern explanations

More scientific views of fossils emerged during the Renaissance. Leonardo da Vinci concurred with Aristotle's view that fossils were the remains of ancient life. For example, da Vinci noticed discrepancies with 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...."

Ichthyosaurus and Plesiosaurus from the 1834 Czech edition of Cuvier's Discours sur les revolutions de la surface du globe

In 1666, Nicholas Steno examined a shark, and made the association of its teeth with the "tongue stones" of ancient Greco-Roman mythology, concluding that those were not in fact the tongues of venomous snakes, but the teeth of some long-extinct species of shark.

Robert Hooke (1635-1703) included micrographs of fossils in his Micrographia and was among the first to observe fossil forams. His observations on fossils, which he stated to be the petrified remains of creatures some of which no longer existed, were published posthumously in 1705.

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. This principle became one of Darwin's chief pieces of evidence that biological evolution was real.

Georges Cuvier came to believe that most if not all the animal fossils he examined were remains of extinct species. This led Cuvier to become an active proponent of the geological school of thought called catastrophism. Near the end of his 1796 paper on living and fossil elephants he said:

All of these facts, consistent among themselves, and not opposed by any report, seem to me to prove the existence of a world previous to ours, destroyed by some kind of catastrophe.

Interest in fossils, and geology more generally, expanded during the early nineteenth century. In Britain, Mary Anning's discoveries of fossils, including the first complete ichthyosaur and a complete plesiosaurus skeleton, sparked both public and scholarly interest.

Linnaeus and Darwin

Early naturalists well understood the similarities and differences of living species leading Linnaeus to develop a hierarchical classification system still in use today. Darwin and his contemporaries first linked the hierarchical structure of the tree of life 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 Darwin wrote On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life, the oldest animal fossils were those from the Cambrian Period, now known to be about 540 million years old. He worried about the absence of older fossils because of the implications on 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 (i.e. phyla) in the oldest known Cambrian fossiliferous strata.

After Darwin

Since Darwin's time, the fossil record has been extended to between 2.3 and 3.5 billion years. Most of these Precambrian fossils are microscopic bacteria or microfossils. However, macroscopic fossils are now known from the late Proterozoic. The Ediacara 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 embedded fossils. 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, for very recent fossils, radiocarbon 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 stromatolites are over 3.4 billion years old.

Modern era

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 Virtual Fossil Museum

Paleontology has joined with evolutionary biology to share the interdisciplinary task of outlining the tree of life, which inevitably leads backwards in time to Precambrian microscopic life when cell structure and functions evolved. Earth's deep time in the Proterozoic and deeper still in the Archean is only "recounted by microscopic fossils and subtle chemical signals." Molecular biologists, using phylogenetics, can compare protein amino acid or nucleotide sequence homology (i.e., similarity) to evaluate taxonomy and evolutionary distances among organisms, with limited statistical confidence. The study of fossils, on the other hand, can more specifically pinpoint when and in what organism a mutation first appeared. Phylogenetics and paleontology work together in the clarification of science's still dim view of the appearance of life and its evolution.

Phacopid trilobite Eldredgeops rana crassituberculata. The genus is named after Niles Eldredge.

 

Crinoid columnals (Isocrinus nicoleti) from the Middle Jurassic Carmel Formation at Mount Carmel Junction, Utah

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 fossilized. This and other data led Stephen Jay Gould and Niles Eldredge to publish their seminal paper on punctuated equilibrium in 1971.

Synchrotron X-ray tomographic analysis of early Cambrian bilaterian embryonic microfossils 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-million-year-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.

Trading and collecting

Fossil trading is the practice of buying and selling fossils. This is many times done illegally with artifacts stolen from research sites, costing many important scientific specimens each year. The problem is quite pronounced in China, where many specimens have been stolen.

Fossil collecting (sometimes, in a non-scientific sense, fossil hunting) is the collection of fossils for scientific study, hobby, or profit. Fossil collecting, as practiced by amateurs, is the predecessor of modern paleontology and many still collect fossils and study fossils as amateurs. Professionals and amateurs alike collect fossils for their scientific value.

Fossils as medicine

These is some medicinal and preventive use for some fossils. Largely the use of fossils as medicine is a matter of placebo effect. However, the consumption of certain fossils has been proven to help against stomach acidity and mineral depletion. The use of fossils to address health issues is rooted in traditional medicine and include the use of fossils as talismans. The specific fossil to use to alleviate or cure an illness is often based on its resemblance of the fossils and the symptoms or affected organ. The use dinosaur bones as "dragon bones" has persisted in Traditional Chinese medicine into modern times, with Mid Cretaceous dinosaur bones being used for the purpose in Ruyang County during the early 21st century.