How Old is Old?

hen did the Earth form?  When did life begin?  When did humans and the other primates first appear?  Reasonably accurate scientific answers to these questions did not develop until the 1960's and 1970's when new radiometric dating techniques were invented that can date samples that are billions of years old.  However, before the 1960's, scholars in many cultures tried to estimate the age of the Earth and of life.

  painting of James Ussher
  James Ussher

In the past, estimates often were based on counts of generations of people in sacred texts.  Manetho click this icon to hear the preceding term pronounced, an ancient Egyptian historian, listed all of the dynasties of pharaohs and gods that reigned down to his time (1380 B.C.).  This made the Earth about 39,000 years old (from our time).  In the early 17th century A.D., Archbishop James Ussher click this icon to hear the preceding term pronounced of Armagh, Ireland accepted the Judeo-Christian Old Testament as being literally true and subsequently determined the age of the Earth by counting biblical generations.  With this method, he calculated that the creation was in 4004 B.C. on October 23 (about 6,000 years ago).

painting of Edmond Halley

Edmond Halley F.R.S. 

With the rapid growth of European scientific knowledge, largely beginning in the late 17th century A.D., it became increasingly clear that the Earth must be much more than a few thousand years old.  Some scientists abandoned the theological approach and began to devise methods for determining the age by measuring natural phenomena.  In 1691, the British astronomer and mathematician Edmond Halley click this icon to hear the preceding term pronounced proposed that if the original oceans were fresh water, one could calculate the minimum age of the Earth by dividing the total amount of salt now present in the oceans by the amount added each year from the world's rivers and streams.   While he actually did not have the full necessary data available, Halley used this approach to conclude that the Earth is minimally 100,000,000 years old.

In 1785, the Scottish geologist James Hutton proposed that different layers of rock came from specific periods of geologic time.  Further, he observed that the layers that were deeper down were older.  In 1799, the English canal and mine engineer William "Strata" Smith expanded on this idea of geologic time by dividing sedimentary rock layers into 6 main divisions based, in part, on the fossil evidence of life that they contained:

 Geologic Era 
Fossil Forms Present
 Cenozoic recent life forms (age of mammals)
 Mesozoic middle life forms (age of reptiles)
 Paleozoic ancient life forms
 Proterozoic early life forms
 Archaeozoic beginning life forms
 Azoic no life forms

These main divisions, or eras, proposed by William Smith became the basic framework for the geologic time scale of the Earth that we still use today.  Smith also created the first detailed geologic map of England.  Since this was the first map of its kind, he is now thought of as the father of geology.

photo of Charles Lyell  
Charles Lyell  
  photo of Charles Darwin

  Charles Darwin

By the mid 19th century, some geologists were estimating the amount of time for the successive changes in animal species found as fossils in the sedimentary rock layers identified by William Smith.  The English geologist Charles Lyell click this icon to hear the preceding term pronounced used this approach in 1867 to estimate that life had been on Earth for about 240,000,000 years.  This estimate was made 8 years after Charles Darwin click this icon to hear the preceding term pronounced published his book On the Origin of Species, which described how natural selection can result in the evolution of new species.

Despite their revolutionary social impacts, these and other early scientific estimates for the age of the Earth and of life were, in fact, far too small.  We now know that the Earth is approximately 4.55 billion years old and that there have been living things on our planet for at least 3.5 billion years.

Modern Methods of Dating the Earth

Modern methods of dating the Earth and the major events in the evolution of life depend on understanding radioactive decay processes.  Certain radioactive isotopes in rocks are the driving mechanisms for reliable "atomic clocks."  For instance, uranium-238 decays ultimately into lead-206. The rate at which this occurs is fixed by the uranium-238 half-life of approximately 4.51 billion years.  In 1905, the American chemist Bertram Boltwood realized that the age of an ancient rock could be determined by carefully measuring the relative amount of these two isotopes.  By 1907, he had dated the oldest rocks that he could find.  Based on this work, he concluded that the Earth must be at least two billion years old.

By the late 1950's, the decay rates of many other elements had been discovered and were beginning to be used for radiometric dating.  Geologists began looking seriously for the earliest piece of the Earth's crust.  We now know that rocks dating to 3.5 billion years are relatively common.  The oldest still intact rock found so far was from the Great Slave Lake region in Canada's Northwest Territory.  It dates to 4.03 billion years.  Still older are zircon crystals found in sedimentary rock from Western Australia.  These date to 4.4 and 4.28 billion years.  However, they are not intact original crust material.  The rock from which they originated eroded and the individual particles became part of the later sedimentary rock.  It is unlikely that we will ever find an intact piece of the original Earth.  Erosion and other geological processes long ago recycled the first crust.  In any case, our planet is almost certainly somewhat older than that solid crust.  The surface of the early Earth was most likely molten rock.

Most astronomers believe that the numerous asteroids and smaller rock objects orbiting our sun primarily between Mars and Jupiter were created at about the same time as the Earth.  Since these bodies lack molten cores and are too small to have atmospheres, it is probable that they are still more or less as they were when they were formed.  Periodically, some of them are pulled out of orbit by comets and crash into the Sun.  On rare occasions, the Earth gets in the way and these rock bodies strike our planet.  Remnants of such meteors that have been dated with multiple radiometric methods range in time from 4.29 to 4.57 billion years, with most of them near the older end of this range.  Based on these dates, it is probable that the Earth is about 4.55 .02 billion years old.  Supporting this estimate are the radiometric dates for rocks brought back from the moon by the Apollo missions of NASA in the early 1970's.  The oldest moon rocks are 4.44 billion years old.  Most astronomers believe that the moon coalesced out of debris from the Earth that was thrown into space by the impact of a massive asteroid or small planet that struck the early Earth.

click this icon in order to see the following video  Radiometric Dating--how known radioactive decay rates are used for dating
This link takes you to a video at an external website.  To return here, you must click
        the "back" button on your browser program.              (length = 1 mins, 39 secs) 

Evolution of Life

time scale of early lifeThe Earth experienced heavy bombardments of asteroids and possibly comets around 3.9 billion years ago.   These devastating impacts of thousands of large space objects very likely went on for 20-200 million years and probably affected the early atmosphere and climates.  It has been suggested that these events could have resulted in the necessary conditions for the appearance of life. 

The earliest signs of life on Earth consist of organic chemicals that were presumably produced by single-celled microscopic organisms.  These chemicals have been found in rock from western Greenland dating to 3.7-3.9 billion years ago.  However, this evidence is not convincing to some researchers because it consists of the chemical markers of life rather than actual cells.  Much more credible have been the discovery of fossil colonies of single-celled microscopic organisms dating to 3.5 billion years ago in Australia and South Africa.  Assuming that the beginning of life on Earth occurred about that time or a bit earlier, we must conclude that our planet was lifeless for its first billion or so years.

By 3-3.5 billion years ago, large colonies of simple single-celled organisms, such as blue-green algae click this icon to hear the preceding term pronounced, were flourishing near the surface of the oceans and seas.  These oxygen producing algae were apparently responsible for the gradual transition of our planet's atmosphere to one suitable for the oxygen breathing creatures that would later evolve.  In addition, more complex single-celled organisms such as protozoa click this icon to hear the preceding term pronounced appeared.  The most advanced creatures were sponges, which are essentially intermediate between single and multi-celled organisms.  Research done by S. Blair Hedges of Pennsylvania State University suggests that the evolution of life accelerated with the appearance of mitochondria around 1.8 billion years ago.  Hedges estimates that the split between what eventually would become the plant and animal lines of evolution occurred about 1.6 billion years ago.  By 1.5 billion years ago, there were simple creatures with up to 10 different kinds of eukaryotic cells.  By 1 billion years ago, their descendents had up to 50 different kinds of cells.

Complex multicellular forms of life are surprisingly recent on our planet. They have existed only for about 600,000,000 years or slightly more.  When these creatures began to appear, 87% of the Earth's time had already gone by and 83% of the total time life has existed had already passed.

time scale of recent life

During the Paleozoic Era click this icon to hear the preceding term pronounced (ca. 570-245 million years ago), there was a comparatively rapid evolution of complex life forms including invertebrates click this icon to hear the preceding term pronounced, vertebrates click this icon to hear the preceding term pronounced, and plants.  In addition, life moved onto the land for the first time.  This dramatic explosion of biological diversity began around 543,000,000 years ago.  During the Mesozoic Era click this icon to hear the preceding term pronounced that followed (ca. 245-65 million years ago), mammals first appeared, but large reptiles dominated the planet.  This was the age of dinosaurs.  With the end of the last dinosaurs by 65,000,000 years ago, the Cenozoic Era click this icon to hear the preceding term pronounced began and more advanced mammals rapidly evolved and became the dominant large animals.  It was only in this last geologic era that primates evolved.  Humans first appeared about 2.5 million years ago.  By then, 99.9% of Earth history had already occurred and most kinds of creatures had become extinct.  

time scale of recent life related to geological eras

The evolution of life has not been a smooth process of ever increasing numbers of species.  There have been major setbacks caused by catastrophic global events, such as ice ages, immense volcanic eruptions, and asteroid impacts.  The result has been at least five episodes of mass extinction and many minor ones.

Approximate Date Of
Extinction Event

  Approximate Duration

  Percent of Species
That Became Extinct

65,000,000 (end of Mesozoic Era)   <1 million   76%
210,000,000 (end of Triassic Period)   3-4 million   80%
250,000,000 (end of Permian Period)   unknown   95-96%
380,000,000 (late Devonian Period)   <3 million   83%
440,000,000 (end of Ordovician Period)   10 million   85%

Source: Gibbs, W. Wayt (2001) "On the Termination of Species", Scientific American Vol. 285, No. 5.

As a result of each of these extinction episodes, there was an opening up of new opportunities for the relatively few surviving species.  As older, often dominant creatures disappeared from the scene, others took advantage of the biological vacuum and rapidly diversified through adaptive radiation.  For instance, the abrupt end of the last dinosaurs 65,000,000 years ago allowed the placental mammals to rapidly evolve into the numerous species lines of today and to become the dominant large animals.  Beginning after this time, primates, rodents, dogs, cats, hoofed animals, and sea mammals all appeared and became major players in most environments around the globe.

click this icon in order to see the following video  Permian-Triassic Extinction (250 million years ago)--geological evidence
        of the greatest loss of life in the history of our planet
This link takes you to a video at an external website.  To return here, you must
        click the "back" button on your browser program.     (length = 2 mins, 21 secs) 

Evolution of the Continents

map of Pangaea

225,000,000 years ago

Map of Laurasia and Gondwana

125,000,000 years ago

The Earth's outer rock shell (the lithosphere click this icon to hear the preceding term pronounced) consists of about a dozen enormous rigid plates and many smaller ones that are more or less constantly moving relative to each other at a rate of a few centimeters a year.  Some are moving slower and others more rapidly.  Most of the Earth's volcanic and earthquake activity occurs where plates come together.   These dynamic processes result in mountain building.   Over tens of millions of years, this tectonic movement  has also caused the continents to travel thousands of miles and to alter their shapes.  It is now known that all of the continents came together and fused early in the time of the dinosaurs.  This super-continent has been named Pangaea click this icon to hear the preceding term pronounced (Greek for "all of the Earth").  It began forming about 285 million years ago.  It was complete by about 210 million years ago and began drifting apart again 10 million years later.

By 250-245 million years ago, Pangaea was massive enough to radically change the world's climates.  The interior of this super-continent would have been far enough from oceans to have extreme seasonal swings in temperature much greater than today.  Intense tectonic movements due to the squeezing together of the continental plates at that time may have triggered massive volcanic activity which would have blocked out the sun with fine ash for years.  Some researchers believe that this would have cooled the Earth's surface sufficiently to trigger a long, devastating ice age.  Supporting this theory is the fact that there was at least a million cubic kilometers of lava that flowed over the surface of what is now Siberia 250 million years ago.  This period coincides with the most complete extinction episode known.  Only about 4% of all plant and animal species survived.  We almost lost life on our planet.  It took about 10 million years for biological diversity to recover following this genetic bottle-necking.  The major animal beneficiaries were the dinosaurs and other reptiles.  By 200 million years ago, the primitive mammals first appeared.


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This page was last updated on Friday, January 14, 2005.
Copyright 1999-2005 by Dennis O'Neil. All rights reserved.
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