A human paleontologist's dream is, of course, to find a complete fossil. But the finest fossil in the world does not bear much scientific importance until we know something about its antiquity. We need to know its age. We need to know where it fits into the biostratigraphic scheme of eras, and periods, and epochs of these geological succession.
Now there are various ways in which we can date, that is determine the age of a specimen. Relative dating is a situation where you can say that an object, a tool or a skeleton, is older or younger than another. For example, if you have an undisturbed stratigraphic succession, the skeleton that is found in the lowest part of the stratification you may assume is going to be older than one in the upper portion which had been laid down more recently. That gives you a relative date but this does not give you a date in solar years. If you want a date that you can define as so many years ago, you have to turn to another practice of dating called absolute dating or chronometric dating.
Chronometric dating means that we are able to estimate the antiquity of an object or a skeleton on the basis of the passage of solar years. One of those methods is very familiar to you I'm sure. And that is radiocarbon dating. You all have heard about radiocarbon dates. These are useful. You can date Egyptian boats by radiocarbon dating. You can date various artifacts that go back into the earlier periods of modern human existence.
However, radiocarbon dating, which is based upon changes in the isotopic structure of carbon-14, has a time depth that's reliable only about 40,000 years. Yes, you can enrich it to about 70,000 years, but 40,000 years is as far back as you can take radiocarbon dating. That's useful for a lot of things, but it's not going to help you if you're dealing with a specimen that may be two or three million years old.
Here you have to find another method of chronometric dating. And many of these are radiometric dating procedures. They have to do with the decay of some elements in materials in relationship to the other materials that are with them, as the decay in the relationship of argon and potassium. These rates of decay are known. We know that they are like clocks. And the degrees of change over time are predictable in solar years.
When we have skeletons where we have these kinds of geological contexts for them, we can date the context of the deposit and therefore have an idea of the antiquity of the specimen itself. This works, of course, if the specimen has not been disturbed or if we're using a method of dating that has a half-life that is long enough to be able to cover the time which is of interest to us.
Some methods of absolute dating are very easy in concept. Tree rings, for example, are used for absolute or chronometric dates because a tree will deposit a ring for every season. And by counting the rings, you know how old the tree is. And by putting together a number of tree ring profiles, you may be able to extend the pattern back for many thousands of years, as has been done in the American Southwest and also in Greece and in Turkey. But even here, you can only take a method that's going back a few thousand years. We don't have tree rings that go back three or four million years.
Another method is to look at the shape of shorelines in lakes. These change every year because of the increase of waterfall in the winter and the resorption of the water in the lake in the summer. And they leave a trace on the shore. It's called a varve. And a varve line will give you a succession of patterns. And if you can date the one that was formed last year, you know that the one below it was formed the year before and so on and provide yourself with a considerable profile of dates. But again, while an absolute date, they are of limited time scope.
Most of the dates that are important to us when we're looking at fossil material of considerable antiquity are those that have to do with potassium-argon, potassium-lead, and other materials where there's a half-life that's much longer and therefore we are in a better position to use a method that is going to extend into that period.
A very interesting method that's been developed more recently has been to measure the distance of the poles of the Earth as they have shifted, because the present position of the North Pole and the South Pole has shifted over many hundreds of thousands of years. And we know something about the periods of time in which those shifts have occurred. Therefore, when we have evidence of polar shifts in rocks, we can estimate this. We're able to say, ah, this was a rock that was changed in this form at a certain period when we knew the poles were in different position. This is called paleomagnetism. And it has proven very useful for a very, very ancient dates.
In dating fossil hominid material, you may also have cultural indications, such as different stone tool industries. You may have heard of hand axes and cleavers. These are stone tools that were made about as long ago as 1.4 million years in Africa. We find them in parts of Asia around a million years ago. Anything that's found in association with these tools then may have an antiquity that great.
But [INAUDIBLE] stone tools and hand axes, as they're called, also extend into much more recent times, even to just a few hundred thousand years ago. These provide dates that are somewhat ambiguous. They're between relative dates and absolute dates. Nonetheless, the artifactual factual record is another way of determining the antiquity of the fossil material which is of importance to you. And it is of importance because it can be dated.
We've received your request
You will be notified by email when the transcript and captions are available. The process may take up to 5 business days. Please contact email@example.com if you have any questions about this request.
Since the time of separation of the evolutionary lines of apes and humans about 5 million years ago, some fossil specimens of the skeletal remains of our earliest ancestors have been preserved and discovered. Putting together the pieces of the puzzle of human biological history is the task of paleontologists, geologists and anthropologists.
In this room we explore how these scientists can reconstruct the past from their studies of the geological contexts in which fossils are found, the dating of the specimens, their comparative anatomy with extinct and living species of our taxonomic order, the Primates, and the lifeways and behavior patterns of the first members of the human family within the Primates.
By looking at reproductions and pictures of this fossil record, including representations of pre-human Primate species, we learn about our prehistoric beginnings among those populations of first two million years of our evolution, as identified as Ardipithecus, Australopithecus and the earliest members assigned to our own genus- Homo habilis and Homo rudolfensis.
This video is part 4 of 13 in the Human Paleontology series.