Ardipithecus ramdidus (Ardi) skull

Once a hominin fossil has been discovered it can be dated by two main ways: 1) by analyzing the volcanic ash around the fossils; 2) by analyzing the other fossils found around the newly discovered hominin bones.

In analyzing the volcanic ash around the fossils. If the ash is older than 1.6 million it can be dated with the potassium-argon method, which dates items by measuring chemical decay. Each volcanic eruption has a unique "fingerprint" and sometimes they scatter ash over a vast area.

In analyzing the other fossils found around the newly discovered hominin bones. Many fossils of the other creatures, such as ancient elephants and rhinos, have been dated before at other locations using the volcanic ash, potassium-argon method described above. If dated fossil are found near the hominin bones it can be said that both species lived around the same time, and hence the homonids samples can be dated. [Source: Kenneth Weaver, National Geographic, November 1985 [┹]

Arguably the the most precise and reliable method for dating samples today is measuring the concentration of radioactive elements and the level of radioactive decay in fossils as well as in the sediments and rocks found side by side with the fossils. The reliability can be improved by having independent laboratories around the world carry out analyses of the same samples, without knowing which samples came from the fossils and sediments being dated and which were 'controls'.

Websites and Resources on Hominins and Human Origins: Smithsonian Human Origins Program humanorigins.si.edu ; Institute of Human Origins iho.asu.edu ; Becoming Human University of Arizona site becominghuman.org ; Hall of Human Origins American Museum of Natural History amnh.org/exhibitions ; The Bradshaw Foundation bradshawfoundation.com ; Britannica Human Evolution britannica.com ; Human Evolution handprint.com ; University of California Museum of Anthropology ucmp.berkeley.edu; John Hawks' Anthropology Weblog johnhawks.net/ ; New Scientist: Human Evolution newscientist.com/article-topic/human-evolution

Dating Techniques and Methods

Volcanic deposits

In the old days Carbon 14, the age of volcanic deposits and age of other fossils found with specimens were the main dating method for dating early man fossils. But each had its shortcomings. Carbon 14 it wasn?t very good for very old specimens; fossils were often found with no volcanic deposits present; and the use of other fossils such as ancient hyenas or antelope wasn’t always so reliable for pinpoint dating because the animals often they lived for a long time on the evolutionary timeline.

These days scientists have a whole host of dating methods at their disposal. They include amino acid racemization, useful in measuring shells and other bicarbonates between 500 and 300,000 years old, and obsidian hydration, used to measure natural glass between 500 to 200,000 years old. These methods are not so useful in measuring the age of fossils and bones but they are useful in measuring the age of objects such as beads and shellfish eaten as food found with fossils and bones.

The geomagnetic polarity timescale method is useful in measuring minerals between 780,000 and 200 million years old in places where there are no volcanic deposits are formed. This method dates objects by measuring the periodic reversals of the north and south magnetic poles, which have occurred at known times and rates.

A dating method used by Richard Bailey at Oxford University measures the amounts of radiation absorbed by sand grains in and around a bone specimen by measuring radioactive isotopes in the sand and comparing that to a sophisticated radiation transport model using data from a CT scan of the specimens. The age of the specimen is determined by calculating the yearly rate at which radiation had been absorbed by the sand grains. [Source: Time magazine, December 18, 2007]

Sometimes geologists can help date objects by measuring the amount of rain wear in cracks, alluvial deposits by streams and glaciers and layers of silt in lakes. Lichenometry is useful in measuring lichens from 100 and 9,000 years old. Dendrochronology is useful in measuring tree rings from zero to 12,000 years old. Tree rings from bristlecone pines provide scale for correcting radio-carbon dating.

Isotopic Techniques

taking a sample for carbon dating and DNA testing

The most common methods of dating fossils and bones rely on the isotopes for dating. Isotopes are atoms of the same element that have different numbers of neutrons and protons in their nuclei. Isotope numbers refer to the combined number of protons and neutrons. Carbon 12, for example, has six protons and six neutrons while radioactive carbon 14 has six protons and eight neutrons. [Source: Carl Zimmer, National Geographic, September 2001]

Radioactive isotopes decay at a predictable rate that can be measured. The rate of decay is often referred to as the half-life of the isotope. Carbon 14 for example has a half life of 5,730 years. This means that after 5,730 years the amount of carbon 14 in an object is reduced by half. After another 5,730 years half of the remaining amount of carbon 14 decays to by half again so that only one quarter of the original amount is left, and so on.

Stable atoms remain stable over time while isotopes decay. When a material is created or, in the case of organic materials, dies the ratio of stable atoms to isotopes is known. By measuring the change in ratio of isotopes to stable atoms an object can be dated. The radioactive decay rates are measured with a spectrometer. Sophisticated mass spectrometers can accurately measure the radioactive decay rates of several kinds of isotopes.

The uranium-lead method is useful in measuring minerals between 1 million and 4.5 billion years old. It dates a mineral by measuring the ratio of amount of uranium and lead produced by the decay of uranium. Uranium-thorium dating, also called thorium-230 dating or uranium series disequilibrium is useful in measuring minerals, shell, bone, teeth and coral between 0 and 400,000 years old.

The rubidium-strontium method is useful in measuring minerals between 60 million and 4.5 billion years old. It dates a mineral by measuring the ratio of amount of rubidium and strontium produced by the decay of rubidium. The potassium-argon method is useful in measuring minerals that around the same age. It dates a mineral by measuring the ratio of amount of potassium and argon produced by the decay of potassium.

Carbon 14 Dating

accelerator mass spectrometer used to measure the carbon 14 in samples

Carbon is useful in measuring minerals, shell, bone, wood, teeth and charcoal between 0 and 40,000 years old. After a living things dies that ratio of carbon 14 isotopes to carbon 12 isotopes decays at a known rate. By measuring the change in ratio of unstable carbon 14 to stable carbon 12 an organic material can be dated. Fossils over 40,000 years have so little carbon 14 left the method is no longer accurate.

Radiocarbon 14 dating was pioneered in the 1940s by University of Chicago chemist Willard Frank Libby, who won the Nobel Prize for his work. Carbon 14 is an unstable radioactive isotope produced when cosmic particles from space slam into nitrogen atoms in the upper atmosphere. Living plants and animals absorb Carbon 14 and other kinds of carbon from carbon dioxide in atmosphere.

When a plant or an animal dies it can no longer absorb carbon. The carbon 14 then begins to revert back to regular carbon (Carbon 12) at a known rate. The amount of Carbon 14 in a sample can thus be used to determine the date the sample died, or in other words when it stopped absorbing carbon.

Carbon 14 has a half life of 5,730 years. This means that after 5,730 years the amount of carbon 14 in an object is reduced by half as the carbon 14 changes to nitrogen 12. After another 5,730 years half of the remaining amount of carbon 14 decays by half again so that only one quarter of the original amount is left — and so on until about 40,000 years when only negligible amounts of carbon 14 is left.

With Carbon 14 dating, a sample of the material to be dated — for example, charcoal from a hearth, a piece of wood from a ship beam, a seed in a strata of soil — is burned and reduced to pure carbon. The ratio of Carbon 14 to Carbon 12 can be measured with a high energy mass spectrometer, revealing the date. Carbon dating can be used to date even minute samples of something such as residue on pots or pigments.

Problems with Carbon Dating

Over the millennia the amounts of Carbon 14 in the atmosphere have not been constant. This means that in certain period a given organism can absorb more or less Carbon 14 depending on how much is in the atmosphere. Fortunately some trees, such as bristlecone pines in California live a long time, and their ages can be accurately measured using tree rings and Carbon 14 levels can be measured in each tree ring and thus a table has been produced that compensates for fluctuations in Carbon 14 levels during each year.

Short-lived plants such as grain are the best for dating. With wood there is always the problem that the tree was cut down long before it was incorporated into a site. In addition, Carbon 14 levels often vary greatly depending on which part of the tree the sample came from: deep inside the tree or on the outside.

Samples can also be contaminated with younger or older carbon brought it by groundwater, earthquakes or carbonate rocks such as limestone. Contamination can usually be eliminated with careful cleaning before the dating process begins. Even when all goes well, the dating does not produce a date but rather a probability that the sample falls within a certain range of dates.

accelerator mass spectrometer schematic for radiocarbon dating

Radiation Exposure Techniques and Amino Acid Racemization

The electron spin method is useful in measuring minerals. tooth enamel, shell and coral between 0 and 1 million years old. This method counts electrons displaced by low-level radiation of uranium thorium and potassium that are found throughout the earth. The more displaced atoms the older an object is.

The fission track method is useful in measuring minerals and natural glass between 500,000 and 1 billion years old. Single-crystal laser-fusion, a relatively new dating process, measures the amount of argon gas released from a laser-melted piece of potassium feldspar, a relatively common volcanic mineral. "Since the argon in the crystal has accumulated at a known rate, the amount of released gas reveals the age of the rock and fossils found nearby." The argon is measured in a gas mass spectrometer.

The amino acid racemization method is useful in measuring shells and other bicarbonates between 500 and 300,000 years old. Amino acids combine to form proteins. They can have a left-handed or right-handed form. For reasons unknown, left-handed amino acids are much more common in nature. Once an amino acid is formed it can spontaneously flip over and became right handed. The rate in which left-handed amino acids switch to right handed ones can be measured to determine a specimen’s age. The switching is not as regular as radioactive decay because heat causes it to speed up and cold slows it down but scientists can make adjustment for these changes by calculating the temperatures and climate in the place where the specimens were found.

Thermolumiscence and Optically Simulated Luminescence

Thermolumiscence counts the number of electrons trapped in the microscopic crystal structure of a burned flint tool or other objects that to have been exposed to early-man-produced heat. By measuring the trapped electrons, the time when an object was last heated can be estimated. The method is useful in measuring minerals and natural glass between 0 and 500,000 years old.

The science behind thermolumiscence is the following: When minerals and natural glass are heated to a certain point radioactive atoms surrounding and buried inside crystals can release particles than can knock electrons out of their orbits. The released electrons sometime get stuck in defects in the crystal structure and over time the crystal fills with electrons at a regular, measurable rate. The trapped electrons are measured by reheating the material. As the trapped electrons escape they release light. By measuring that light scientists can count the trapped electrons and determine the age of the material.

Optically simulated luminescence is used to determine when minerals such as quartz are buried under sand or sediments by determining when they were last exposed to sunlight. This method operates under the same principals as thermolumiscence: by measuring the trapped electrons, the time when an object was last exposed to sunlight can be estimated. The trapped electrons are measured by firing beams of photons at the object. As the trapped electrons escape and return to their atoms they release heat. My measuring the heat scientists can count the trapped electrons and determine the age of the material.

Optically simulated luminescence is useful in measuring minerals and natural glass between 0 and 500,000 years old. The trick is to find objects that have not been exposed to sunlight and prevent them from being exposed to sunlight. Just of few seconds of exposure to sunlight can cause the trapped electrons to break from the crystals and return to their original state. Scientists who rely on this methods can not look for fossils and objects in the normal way in the sunlight. They hammer hollow, stainless steel cylinders into the sand and capped them and later examine their finds in a darkroom and fire beams of photons their samples to release the trapped electrons.

Genetic Dating and the DNA Clock

Extracting Neanderthal DNA

Katherine Sharpe wrote in Archaeology: For years, archaeologists and geneticists have been troubled by the fact that their time lines for key events in human evolution don’t always match up. While archaeologists rely on the dating of physical remains to determine when and how human beings spread across the globe, geneticists use a DNA “clock” based on the assumption that the human genome mutates at a constant rate. By comparing differences between modern and ancient DNA, geneticists then calculate when early humans diverged from other species and when human populations formed different genetic groups. [Source: Katherine Sharpe, Archaeology, December 17, 2012 +||+]

“The DNA clock is a powerful tool, but its conclusions—for example, that modern humans first emerged from Africa about 60,000 years ago—can disagree with archaeological evidence that shows signs of modern human activity well before that date at sites in regions as far-flung as Arabia, India, and China. +||+

“Now, new work, based on observation of the genetic differences between present-day parents and children, suggests that the genetic clock may actually run about twice as slowly as previously believed, at least for the last million years or so of primate history. In their review paper in the journal Nature Reviews Genetics, Aylwyn Scally and Richard Durbin of the Wellcome Trust Sanger Institute in Hinxton, England, propose much earlier dates for watershed events in human evolution, which could help bring the genetic and archaeological records in line. For instance, a slower clock places the migration of modern humans out of Africa at around 120,000 years ago, which is more consistent with archaeological evidence. +||+

“The revised clock also supports archaeological signs of modern human activity from more than 60,000 years ago at sites such as Jwalapuram, India (“Stone Age India,” January/February 2010), and Liujiang, China—evidence that has often been dismissed by geneticists as impossible. While more work is needed to confirm the findings, Scally says that archaeologists who work on such sites should be excited: “It can no longer be said that the genetic evidence is unequivocally against them.”“ +||+

Problems with Genetic Dating

Bridget Alex wrote in The Guardian: “For the past several years, there have been two main genetic methods to date evolutionary divergences - when our ancestors split from Neanderthals, chimpanzees, and other relatives. The problem was, the results of these methods differed by nearly two-fold. By one estimate, modern humans split from Neanderthals roughly 300,000 years ago. By the other, the split was closer to 600,000 years ago. Likewise, modern humans and chimps may have diverged around 6.5 or 13 million years ago. [Source: Bridget Alex, The Guardian, December 22, 2016. Bridget is a postdoctoral fellow in the department of human evolutionary biology at Harvard |=|]

“Puzzled by this wild disagreement, researchers with diverse expertise have been studying it from different angles. Their combined discoveries, recently reviewed here and here, have shed light on how genetic differences accumulate over time and have advanced methods of genetic dating. And if you’re in suspense, yes, they’ve also pinned down important events in our evolutionary timeline. Everyone alive today seems to share ancestors with each other just over 200,000 years ago and with Neanderthals between 765,000-550,000 years ago.”

Key to Genetic Dating: You Need to Know the Rate

Bridget Alex wrote in The Guardian: “Go back in time and you’ll find a population of Homo sapiens who were the ancestors of everyone living today. Go back farther and our lineage meets up with Neanderthals, then chimps, and eventually all primates, mammals, and life. In order to date these evolutionary splits, geneticists have relied on the molecular clock - the idea that genetic mutations accumulate at a steady rate over time. Specifically this concerns mutations that become neutral substitutions, or lasting changes to letters of the genetic code that do not affect an organism’s chances of surviving and reproducing. |=| “If such mutations arise clocklike, then calculating the time since two organisms shared common ancestors should be as easy as dividing the number of genetic differences between them by the mutation rate - the same way that dividing distance by speed gives you travel time. [Source: Bridget Alex, The Guardian, December 22, 2016 |=|]

“For decades, anthropologists used fossil calibration to generate the so-called phylogenetic rate (a phylogeny is a tree showing evolutionary relationships). They took the geologic age of fossils from evolutionary branch points and calculated how fast mutations must have arisen along the resulting lineages. For example, the earliest fossils on the human branch after our split with chimps are identified by the fact that they seem to have walked on two legs; bipedalism is the first obvious difference that distinguishes our evolutionary lineage of hominins from that of chimps. These fossils are 7-6 million years old, and therefore the chimp-human split should be around that age. Dividing the number of genetic differences between living chimps and humans by 6.5 million years provides a mutation rate.

“Determined this way, the mutation rate is 0.000000001 (or 1x10-9) mutations per DNA base pair per year. Applied to genomes with 6 billion base pairs, that means, over millions of years of chimp and human evolution, there have been on average six changes to letters of the genetic This rate can be used to date evolutionary events that are not evident from fossils, such as the spread of modern humans out of Africa. |=|

“But genetic dating got messy in 2010, when improvements to DNA sequencing allowed researchers to determine the number of genetic differences between parents and their children. Known as pedigree analysis, this provides a more direct measurement of the current mutation rate within one generation, rather than an average over millions of years. Pedigree analysis counts 60-some mutations every generation; that converts to a rate approximately half the phylogenetic estimate—meaning evolutionary events would be twice as old.

Genetic Dating Rate Can Vary Based on Species, Sex and Mutation Type

Bridget Alex wrote in The Guardian: “Resolving this disagreement propelled researchers to reassess and revise their starting assumptions: How accurately were they counting the small number of differences between genomes of parents and children? Were fossils assigned to the correct branches of the evolutionary tree? And above all, how constant is the molecular clock? [Source: Bridget Alex, The Guardian, December 22, 2016 |=|]

“It turns out that among primates, the molecular clock varies significantly by species, sex, and mutation type. A recent study found that New World monkeys (i.e. monkeys of the Americas like marmosets and squirrel monkeys) have substitution rates about 64 percent higher than apes (including humans). Within apes, rates are about 7 percent higher in gorillas and 2 percent higher in chimpanzees, compared to humans. But even among humans, mutation rates differ, particularly between the sexes with age. As fathers get older, they gain about one additional mutation per year in the DNA they can pass on to children. Mothers, on the other hand, accumulate considerably fewer mutations with each passing year. |=|

“These species and sex differences make sense when you consider how mutations form. Most heritable mutations occur from mistakes when DNA copies itself in the germline, or cells leading to eggs and sperm. The number of times germline DNA has to copy itself depends on developmental and reproductive variables including age at puberty, age at reproduction, and the process of sperm production. |=|

“These traits vary across primates today, and certainly varied over primate evolution. For instance, average generation times are six years for New World monkeys, 19 years for gorillas, 25 years for chimps, and 29 years for humans. And those extra mutations as fathers get older? Sperm are produced continuously after puberty, so sperm made later in life are the result of more rounds of DNA replication and opportunities for replication errors. In contrast, a mother’s stock of eggs is formed by birth. The small increase with maternal age could be due to mutations from DNA damage, rather than replication errors.

Tweeking and Improving Genetic Dating

Bridget Alex wrote in The Guardian: “It’s now clear that one mutation rate cannot determine the dates for all divergences relevant to human evolution. However, researchers can secure the timeline for important evolutionary events by combining new methods of genetic dating with fossils and geologic ages. Innovative computational methods have incorporated reproductive variables into calculations. By taking into account ages of reproduction in both sexes, age of male puberty, and sperm production rates, researchers have estimated split times that accord with the fossil record. [Source: Bridget Alex, The Guardian, December 22, 2016|=|]

“Another new approach has analysed mutations that are mainly independent of DNA replication. It seems that certain classes of mutations, related to DNA damage, do behave more clocklike. And some researchers have focused on ancient DNA. Comparing human fossils from the past 50,000 years to humans today, suggests a mutation rate that agrees with pedigree analysis. |=|

“At least one evolutionary split was pinned down in 2016, after ancient DNA was extracted from 430,000 year-old hominin fossils from Sima de los Huesos, Spain. The Sima hominins looked like early members of the Neanderthal lineage based on morphological similarities. This hypothesis fit the timing of the split between Neanderthals and modern humans based on pedigree analysis (765,000-550,000 years ago), but did not work with the phylogenetic estimate (383,000-275,000 years ago). |=| “Where do the Sima hominins belong on our family tree? Were they ancestors of both Neanderthals and modern humans, just Neanderthals, or neither? DNA answered this definitively. The Sima hominins belong to the Neanderthal branch after it split with modern humans. Moreover, the result provides a firm time point in our family tree, suggesting that the pedigree rate works for this period of human evolution. |=|

“Neanderthals and modern humans likely diverged between 765,000-550,000 years ago. Other evolutionary splits may soon be clarified as well, thanks to advances brought about by the mutation rate debates. Someday soon, when you see a chimp, you may be able to salute your great, great… great grandparent, with the correct number of “greats.” |=|

Image Sources: Wikimedia Commons

Text Sources: National Geographic, New York Times, Washington Post, Los Angeles Times, Smithsonian magazine, Nature, Scientific American. Live Science, Discover magazine, Discovery News, Natural History magazine, Archaeology magazine, The New Yorker, Time, BBC, The Guardian, Reuters, AP, AFP and various books and other publications.

Last updated September 2018

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