Photo credit: Francois Gohier


What is a fossil?
Fossilization is an exceptionally rare occurrence. After death, organisms tend to decompose quickly. What scavengers like vultures and hyenas leave behind, flies, ants, worms, and bacteria quickly consume. Scientist Olivia Judson provides this good example of what happens to an adult male gorilla in the tropical rainforests of the Congo; "An adult male gorilla— all 330 pounds of him—will be reduced to a pile of bones and hair within 10 days of his death. Within three weeks, there will be nothing left but a few small bones."

A fossil normally preserves only a portion of an organism, usually that portion that was partially mineralized during life, such as bones and teeth. Trace fossils are the marks left by a living organism, such as feces, footprints or impressions of feathers or leaves. Organisms usually need to be covered by mud, sand, tar or some other sediment as soon as possible or frozen or dessicated (dried out) for fossilization to occur.

How old does something have to be to be a fossil?
Fossils, by definition, are the remains or traces of organisms that lived at least 10,000 years ago. This date marks the end of the Cenozoic Era and the Pleistocene Period on the geologic time scale. Paleontologists consider materials younger than 10,000 years ago “recent”.

How old is the oldest fossil on earth?
The oldest uncontested fossils on earth are 2 billion year-old stromatolites in Minnesota, Wisconsin, and Ontario. Composed of layers of sediments laid down by colonies of cyanobacteria, stromatolites still exist, but are quite rare today.

How old is the earth?
The earth itself is around 4.5 billion years old. (USGS)

How old is the sun?
The sun formed around 4.6 billion years ago. (See NASA Web Archive for more information)

How do researchers determine radiocarbon dates? How accurate are they?
Radiocarbon dates come from organic matter that contain the element carbon. Ice cores, pollen, wood, hair, bone, shell, bird egg shell, and coral are some of the things datable by this technology which depends on measuring the atomic decay rate of carbon isotopes. Material older than about 50,000 years can’t be dated with radiocarbon techniques because too little of the original radioactive material remains. Modern labs can measure samples as small as 100 mg (0.003 oz) to a precision of ±16 years (Radiocarbon Web Info)

Can all fossils be dated with radiocarbon dates?
Most fossil are dated by decay of isotopes such as Uranium 235, Potassium 40, and Rubidium 87 that have much longer half-lives than carbon 14. These isotopes aren’t found in the fossils themselves, but in the rock encasing the fossils

Who decides what the geologic time scale should say?
Geologists in the International Commission on Stratigraphy make proposals based on the most recent studies to constantly update the geologic time scale of the world’s rocks. This chart uses both radiometric, relative comparisons, fossils, and reversals in the earth’s magnetic field to order events in the history of the earth.


present to 65 million years ago (mya)

Holocene (Recent)

Today - 10,000 yrs. ago

Beginning of human agriculture; continents elevate (rebound) after Pleistocene ice melts


10,000 yrs ago to 2.6 mya

Repeated ice ages; Homo sapiens and Homo neanderthalensis; Smilodon and other large mammals extinct at end


2.6 to 5.3 mya

Sea cows abundant; Isthmus of Panama connects North and South America


5.3 to 23.7 mya

Giant shark Megalodon is apex predator


23.7 to 33.9 mya

Grasslands spread, forests recede


36.6 to 55.8 mya

Early whales, horses, primates, bats evolve; climate very warm, then cooler


55.8 to 65 mya

Mammals flourish after end-Cretaceous extinctions

65 to 251 mya


65 to 144 mya

First flowers attract pollinators; Tyrannosaurus-rex and other non-bird dinosaurs, pterosaurs, ammonites extinct at 65 mya


144 to 206 mya

First birds; super-continent Pangea separates; giant herbivorous dinosaurs (sauropods) tower over small mammals


206 to 248 mya

First dinosaurs and first mammals (both small) occupy Pangaea

251 tp 542 mya


248 to 290 mya

Reptiles dominate land; continents come together, forming Pangea; major extinction at end wipes out 95% of all life


290 to 354 mya

First reptiles leave fossil footprints; large amphibians common


354 to 417 mya

Land colonized by first amphibians; sharks and rays appear in oceans


417 to 443 mya

First reefs with corals, bony fish, clams, snails appear in oceans; first insects and spiders evolve on land


443 to 490 mya

First land plants invade warm wet lowlands


490 to 543 mya

First vertebrates are the jawless fishes; invertebrate trilobites dominate oceans

542 to 4,500 mya


543 to 2,500 mya

First microscopic organisms in oceans


2,500 to 3,800 mya

Much volcanic activity; first small continents


3,800 to 4,500

Oldest known rocks on earth

4,600 mya

Earth forms


Important Web Resources:

About the Photograph:

Living stromatolites in Shark Bay World Heritage Site of Western Australia are built by colonies of bacteria that draw energy from the sun and carbon dioxide, releasing oxygen (photosynthesis). A sticky goo that surrounds the bacteria allows precipitation of minerals and entrapment of sediments, making a bulbous structure. Scientists are still finding a few new sites where they live today but they were quite abundant on earth some 2 billion years ago, and perhaps earlier. Geologists credit stromatolite activity for creating the earth's first atmospheric oxygen and making animal life possible.

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