Time IS of the Essence
Although I have learnt that fossil evidence is not essential to prove evolution by natural selection, it does nevertheless provide visual, tangible support. In order then to produce a chronological history of life on Earth, we must have reliable methods of dating fossils.
Relative age can be determined by recording which strata it lay in. A cross section through a cliff edge, for example, may show the different strata. Those nearest the top being the youngest, those further down being older. The age of fossils found in the different layers will therefore also be older or younger. But to determine the absolute age of a fossil more ingenious methods need to be used.
In this day and age there are many corroborating dating methods that overlap each other. For example by counting tree rings we can work out the age of a tree when it stopped growing. But most trees do not grow more than a hundred years or more so what use is that? Well, tree rings are like finger prints, the pattern of their rings being determined by how seasons vary each year. So by comparing tree rings with over lapping ages it is possible to date a tree to the nearest year by going back many generations beginning from when we know the tree was alive. But that will still be a difficult process for aging fossils that might be tens of thousands, millions or even billions of years old.
Radioactive clocks provide methods often used for this, but how do they work?
Firstly, a bit about atoms and elements. Three kinds of particle enter into the makeup of an atom, electrons, protons and neutrons. The model used to visualise an atom is of a nucleus made up of protons and neutrons with electrons orbiting it. Every element has the same number each of protons and electrons which defines its atomic number, for example, copper has the atomic number 29 because there are 29 protons in the nucleus and 29 electrons.
However there can be different versions of each element which are called isotopes. What makes them different is the number of neutrons in the nucleus.
For example Carbon can come in 3 versions Carbon-12 which is the most common which has an equal number of neutrons and protons, but there is also Carbon-13 and Carbon-14 which have 1 or 2 more neutrons respectively. These extra neutrons make the atom unstable, it becomes radioactive, but it does not remain unstable. Over time Carbon-14 will steadily decay into Nitrogen-14. Nitrogen has the same atomic mass as Carbon except it has 7 Protons and 7 Neutrons while Carbon-14 has 6 protons and 8 Neutrons.
All living matter absorbs Carbon either through the food it eats or through photosynthesis and because the ratio of Carbon-12 to Carbon-14 in any organic matter is fixed, the moment organic matter dies it stops absorbing Carbon. If then, for example, it is encased in volcanic lava which solidifies, the Carbon content is trapped and the clock is set to zero. As time goes by the trapped Carbon-14 will then decay into Nitrogen while the amount of Carbon-12 will remain the same. The time it takes for half the amount of Carbon-14 to decay into Nitrogen is called its half-life. This takes around 6000 years. For half of what is left to decay will take another 6000 years and so on. The ratio of Carbon-12 to Carbon-14 left in the artefact, compared to when it died, will therefore provide the age of the item, to within 100 years.
Carbon is not the only element that has unstable isotopes. Some have very short half-lives others very long ones thus providing the appropriate range for whatever we are dating.
A timeline for the different ages of the Earth is now pretty well established. It is interesting that fossils from a more recent age have never been found in an earlier strata.