The fossilised remains of a hominid, Australopithecus sediba, found in a cave in South Africa last year have been investigated further to get a more precise estimate of their age. It is thought that the remains are 1.98 million years old, plus or minus 3000 years.

The importance of dating A. sediba as precisely as possible is so that researchers can identify its place in the history of human evolution. Humans all belong to the genus Homo, as in our species Homo sapiens, and it is thought that A. sediba is a direct ancestor of the Homo genus. So these remains could be the ancestors of all the Homo sapiens, Homo neanderthalensis, Homo erectus etc that have ever walked the Earth.

Initially the remains were dated to within plus or minus 200,000 years so the current margin of error of 3000 years is a significant improvement. To put this in a more comprehensible time-scale, it’s like saying that I had breakfast yesterday some time around 08.00, it could have been 06.00 or it could have been as late as 10.00. Compare this to saying that I had breakfast yesterday some time around 10.00, it could have been 09.58 or it could have been as late as 10.02.

The key to this improvement has been the use of two techniques called uranium-lead dating and paleomagnetic analysis. Both are methods that analyse the rock that the fossils are found in. The first relies on nuclear reactions of uranium atoms, and the second is concerned with the orientation of magnetic particles in the rock.

The uranium-lead method works because the rate at which uranium atoms turn into lead atoms is known very precisely. The technique uses two types (isotopes) of uranium, U-238 and U-235. Both of these atoms are prone to breaking and forming small atoms. It is impossible to predict when an individual atom will break-up, or decay, but it is possible to predict how many will decay in time if you have millions of them. For instance, we know that it takes 700 million years for half of the U-235 atoms to decay. This isotope of uranium will decay to form a lighter atom called thorium. So how is this used, and where does the lead come in?

The products of the uranium decay will go on to decay themselves, as will their products and so on. U-238 will go through 14 stages of decay before the atom involved becomes an isotope of lead called Pb-206.  U-235 has 11 stages of decay before it becomes Pb-207.

Scientists use this by finding out the amounts of uranium and lead that the rocks contain. Let’s take a mineral such as zircon (ZrSiO₄) as an example of how this information is used. It doesn’t contain any uranium or lead itself, but if there are any uranium atoms around as the zircon is being formed, then these will be incorporated into the crystal structure of the mineral. The opposite will happen to lead. Lead is rejected by the crystal and so as zircon is formed it may contain traces of uranium, but it will not contain any lead.

Over the course of time the uranium atoms will decay, becoming lead atoms. When the ziron is analysed, the amount of uranium compared to lead can be used to give scientists a date as to how long the rock has been in existence for.

In the case of the A. sediba remains in South Africa, these were found surrounded by deposits of flowstone in a cave. Flowstone is formed by running water losing carbon dioxide and thereby laying down deposits of carbonate minerals. This is the same process that  forms stalactites and stalagmites. It is the uranium-lead content of the flowstone that has been found that has been used to date the A. sediba fossils.

The other technique used to date the flowstone is paleomagnetic analysis. In this case it is tiny magnetic particles trapped in minerals that provide the information required. As the minerals are forming, any magnetic particles are free to move. These will line-up with the Earth’s magnetic field just like a compass needle would. When the mineral is formed these particles are locked in position and so form a record of the direction of the Earth’s magnetic field at the time.

The field direction has varied a lot over the eons and has on occasion been completely the reverse of the direction today such that the magnetic north pole is where today’s south pole is. As the mineral deposits build up, so does a pattern of the changes in field direction, and this can be used to date the rock.

Using the two techniques in this manner has given researchers a date for the A. sediba remains with a stunning level of precision and means that the cave where they were found is now the best dated fossil site in the world.

See the original article at Fossil discovery could be our oldest human ancestor