It is fairly easy for all of us to grasp events that happened a generation or two ago. Quite often, we heard stories from our parents and grandparents relating to these. Most people don’t have problems either with things that occurred centuries earlier. We know Napoleon existed, and so did Julius Caesar. Babylon was a reality as well, but as we keep going further back in time, questions arise. How do we know that Lucy lived 3.12 million years ago?
The approaches used in dating the fossil remains of early humans vary from region to region. In Africa, there are regions where we can use volcanic ash to help date the layers in which, or between which early human fossils have been found. This technique, known as paleomagnetic dating, has been used in East Africa, including on the site where Lucy was found. Volcanic ash layers, unfortunately, are not available in Southern Africa. Here another technique has been used, known by its fancy name as biostratigraphy.
Biostratigraphy, or the dating of layers of rocks and dirt based on the presence of animal fossils, relies on the known and dated presence of specific animal species. If animal remains are found in association with human remains, then one can compare the animal remains against those found elsewhere, specifically those found in areas where other dating techniques are available. Scientists prefer working with animal species that had a great geographic range and with short lived history. (The latter is important so that the period of time during which they could be incorporated in the sediment is relatively narrow and therefore easier to date.)
These approaches reflect flexibility on the part of scientists. You may be aware of a technique that would help you date a fossil, but if you do not have the right material (like volcanic ash) to work with, then you have to resort to other approaches, such as biostratigraphy.
A dating technique that is very well known is radiocarbon or C14 dating. This does not apply here. Radiocarbon dating can only help us go back in time 50,000 years ago, give or take a few thousand years. Since Lucy lived well before that, radiocarbon is of no use to us in this case.