When did our earliest ancestors diverge from the apes and begin walking on two legs? When did the early hominids first use stone tools? And when did Homo sapiens migrate out of Africa? Answering these key evolutionary questions depends on accurate dating techniques.
“Telling geological time is one of the most important attributes to this kind of study because without being able to put fossils in their precise time frame, the fossils are just floating in our understanding of where they fit in the evolutionary sequence,” says University of Toronto geologist Bob Walter. He dated Lucy, the famous 3.2-million-year-old hominid fossil. He’s also head of an international team that’s found hundreds of hand axes and animal fossils in the badlands of Eritrea in northeast Africa.
The unusually rich find is thought to be about a million years old. But how do scientists know that?
Law of superposition
In the early days of geology and paleontology – the late 1700s – scientists used what are called relative dating techniques. The first and most simple technique is the law of superposition, which simply states that in a sequence of sedimentary rock, the rocks in the upper layers of sediment are younger than the rocks at the bottom. That’s because sedimentary layers stack up over time – newer layers building on top of the older ones like a layer cake.
“So, if you go to a sequence of rocks that are horizontally bedded and you find fossils in that sequence, then you can look at how those fossils change from the bottom to the top, or from oldest to youngest,” explains Walter. “You may not know exactly what date you’re talking about, but you can say we’re dealing with a particular period because it falls in this geological strata and it’s older than these materials that are found above it.”
The law of superposition doesn’t pin actual dates to fossils, but that didn’t stop our early scientists from trying, says Walter. By ‘guess-timating’ how long it would take for sedimentary layers to accumulate, our early geologists and paleontologists “dated” their discoveries. This caused a lot of controversy over dating relics, says Walter. It took the discovery of radioactivity and 60 years of technological innovation to get scientific accuracy – and agreement – in geological dating.
Discovery of radioactivity
In 1896, French scientist Henri Becquerel discovered an invisible, penetrating radiation emitted spontaneously by uranium. Six years later in 1902, two British physicists, Ernest Rutherford and Frederick Soddy, showed that radioactive elements also spontaneously transform into another element. For example, potassium 40, a common solid element found in rocks, changes into argon 40, a gas. With the subsequent advent of nuclear physics, scientists learned that each radioactive element has it own particular rate of radioactive decay as it changes into another element – giving scientists a reliable geological clock. Fossils tend to lose argon by diffusion over time, but by measuring the amount of argon gas in the rock surrounding a relic, scientists can calculate its age.
“So we can take a rock, melt it to extract the argon from it, and count the atoms,” explains Derek York, a geochronologist at the University of Toronto, “Knowing the rate at which potassium produces argon, we can say how long that rock must have been there.”
Potassium-argon dating was first used to date Zinjanthropus, an Australopithecine discovered by Mary Leakey in 1959 at Olduvai Gorge, Tanzania. By dating the volcanic rocks that surrounded the fossil, scientists found that the hominid was 1.8 million years old.
Using lasers to melt rocks
Dr. York made the next breakthrough in dating technology in 1981. He and his colleagues at the University of Toronto came up with a way to use a laser beam to melt individual crystals of volcanic rock, that has now become standard in laboratories around the world.
“It melts the rock beautifully,” says York. ” The great thing is it gives you much lower contamination.”
As York explains, a given sample of volcanic rock crystals will have contaminants – crystals of a much younger or older age that look exactly the same as the majority of the crystals. The contaminants skew the average age of the sample. By melting the crystals individually, scientists can screen out any contamination that might throw off the age.
How accurate is the technique? “You should be able to say that Lucy, for example, is 3.2 million years old, with an error of the order of one percent, or 30,000 years,” says York.
Walter and his team are still in the early stages of dating the stone tools found in Eritrea, and have made an estimate of their age based on the animal fossils they found – what are called index fossils.
“Certain species of pig or elephant are very good index fossils,” explains Walter. “Pigs evolve quickly, geologically speaking. If a species of pig is identified and dated at one site, and then that same species is found at another, undated site, then the age of the new site can be estimated by correlation.”
Pig fossils found at the site, like the pig tooth shown here, place the age of these sediments somewhere between half a million and one-and-a-half million years old.
The next thing to do is find enough volcanic rocks mixed into the fossil-bearing sedimentary rocks to come up with exact dates.
“We haven’t found the best material to date yet,” says Walter, “but that’s something for next field season.”