Fossils are NOT dated directly. They are dated indirectly. They date the rocks, not the bones. Is it possible that the rocks are much older than the bones? If a human dies and his skeleton by chance becomes fossilized (or any part of it) it is highly probable that the rocks near the remains are much older than the bones.
So let’s say I am hiking in the wilderness and fall over dead near some rocks and I’m lucky enough to be buried by a landslide so that my remains become fossilized. Then 1 million years from now an archaeologists comes along and discovers my fossil remains and dates the rocks (because there is still no technique to date the bones) at 3 million years old he concludes that my fossil remains are 3 myr. BUT they aren’t 3 myr! My fossil remains are only 1 myr!!
However, I’m sure this future archaeologist would use more than one technique to date my fossil remains. He might use Relative Dating in which fossils and layers of rock are placed in order from older to younger. But, things usually aren’t that simple because even though theoretically these layers should be from older (at the bottom) to younger (at the top) things happen in nature and sometimes layers get all mixed up. So that might mean the layer in which my fossil remains are found is NOT the layer in which I actually died. And the layer may be 3 myr but I only died 1 mya. So does the future archaeologist date me at 3 myr or 1 myr? He’d have to go by the rock layers and conclude that I died 3 mya.
The future archaeologist might also use Radiometric Dating. Up until the middle of the last century scientists only had the Relative Dating technique to date fossils by so there was no way they could calculate absolute age in years for any fossil or rock layer. When scientists began to learn about nuclear decay of radioactive elements they realized the rate of decay happens at a predictable rate. The traces of these radioactive elements found in certain rocks like lava rock and tuff (compacted volcanic ash) could be analyzed chemically and that this process would yield a date (age) in years of those rocks. This is Radiometric Dating but there is still a problem with dating fossil remains because the rocks NOT THE FOSSIL is being dated. So, again, our future archaeologist will conclude by this method too that my fossil remains are 3 myr even though they are really only 1 myr!
So let’s say the future archaeologist puts these two methods together in an attempt to date my fossil remains. He would likely conclude that the landslide happened about 1 mya and since my fossil remains were found in that layer he would accurately date my fossil remains at 1 myr which would be correct. However, if I died before the landslide then my fossil remains would be under that layer of landslide meaning my fossil remains would be OLDER that 1 myr. And if my fossil remains were found above the landslide layer that would mean my fossil remains were YOUNGER than 1 myr.
But what would happen if all those layers got all mixed up by some natural catastrophe? Let’s say like a really big flood or something. And let’s say our future archaeologist found my fossil remains in the layer dated at 500 kya. He might date my fossil remains at 500 kyr even though they are actually 1 myr!
To be fair to this subject there are several newer dating techniques that I think date more accurately than some of the older ones.
So what’s my point?
As mentioned above we date the rocks not the fossil bones and it would stand to reason that the bones are younger than the rocks as I do not think the rocks were created the moment I died. So we need to be careful about dating and dates because dates can be easily deceiving.
Below is a link that shows some of the various dating techniques used to date fossil remains of early humans et al.
One thing that we can do now that we couldn’t do in the past is use dating methods such as Thermo-luminescence Dating, Optically stimulated luminescence, and Electron spin resonance all of which measure the amount of electrons that get absorbed and trapped inside rock OR TEETH over time in fossil remains. Another interesting dating technique is known as the Molecular Clock Technique (also known as the Molecular Genetic Clock method) which compares the amount of genetic difference between living organisms (such as modern humans) and archaic organisms (such as ancient human ancestors like Neanderthals) and computes an age based on well tested rates of genetic mutation over time. However, the problem with this method is that genetic material like DNA decays rapidly over time so we can’t date very old fossils with this method such as H. habilis. The MC technique is pretty useful for determining how long ago living species hared a common ancestor based on their DNA.