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It is based on the decay rate of the radioactive carbon isotope 14 C, a form of carbon taken in by all living organisms while they are alive. Before the twentieth century, determining the age of ancient fossils or artifacts was considered the job of paleontologists or paleontologists, not nuclear physicists. By comparing the placement of objects with the age of the rock and silt layers in which they were found, scientists could usually make a general estimate of their age.
However, many objects were found in caves, frozen in ice , or in other areas whose ages were not known; in these cases, it was clear that a method for dating the actual object was necessary. In , the American chemist Bertram Boltwood — proposed that rocks containing radioactive uranium could be dated by measuring the amount of lead in the sample.
This was because uranium, as it underwent radioactive decay , would transmute into lead over a long span of time.
Thus, the greater the amount of lead, the older the rock. Boltwood used this method, called radioactive dating , to obtain a very accurate measurement of the age of Earth. While the uranium-lead dating method was limited being only applicable to samples containing uranium , it was proved to scientists that radioactive dating was both possible and reliable.
The first method for dating organic objects such as the remains of plants and animals was developed by another American chemist, Willard Libby — He became intrigued by carbon — 14, a radioactive isotope of carbon. Carbon has isotopes with atomic weights between 9 and The most abundant isotope in nature is carbon — 12, followed in abundance by carbon — Among the less abundant isotopes is carbon — 14, which is produced in small quantities in the earth 's atmosphere through interactions involving cosmic rays.
In any living organism, the relative concentration of carbon — 14 is the same as it is in the atmosphere because of the interchange of this isotope between the organism and the air. This carbon — 14 cycles through an organism while it is alive, but once it dies, the organism accumulates no additional carbon — Whatever carbon — 14 was present at the time of the organism's death begins to decay to nitrogen — 14 by emitting radiation in a process known as beta decay. The difference between the concentration of carbon — 14 in the material to be dated and the concentration in the atmosphere provides a basis for estimating the age of a specimen, given that the rate of decay of carbon — 14 is well known.
The length of time required for one-half of the unstable carbon — 14 nuclei to decay i. Libby began testing his carbon — 14 dating procedure by dating objects whose ages were already known, such as samples from Egyptian tombs. He found that his methods, while not as accurate as he had hoped, were fairly reliable. Libby's method, called radiocarbon or carbon — 14 dating, gave new impetus to the science of radioactive dating.
Using the carbon — 14 method, scientists determined the ages of artifacts from many ancient civilizations. Still, even with the help of laboratories worldwide, radiocarbon dating was only accurate up to 70, years old, since objects older than this contained far too little carbon — 14 for the equipment to detect.
Being a recently appointed young chap in this field, I was given to him as a personal assistant. And after three or four months of that, he said that just before he left Harvard University to come out to Australia his personal assistant there had resigned.
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Would I like to come back and work for him, with half time to do courses at the university! In Australia we were looking at termite protozoa, the cellulose-digesting organisms in the gut of the termite. Cleveland had made a huge reputation by being the first person to learn how to get rid of the protozoa, thereby showing that they were essential for the digestion of cellulose.
That is a difficult thing to digest and these fascinating organisms do it for most of the groups of termites. We made slides of these things, and later — back in Cambridge, Massachusetts — we studied them in great detail.
Cleveland asked me to go to South Africa on my way to Massachusetts. He had come to Australia via New Zealand, and had collected a particular group of termites which has an interesting distribution: New Zealand, the east coast of Australia and South Africa. This is a Gondwanaland distribution, from when the ancient continents were joined through the Antarctic, South Africa and so on.
He wanted to collect the ones in South Africa but he didn't want to travel there because he had his wife and family with him. Instead I collected these things, in a small South African town called George. And I remember very well that the director of the little forestry school there was very kind and gave me not only accommodation but also, after dinner, a tumbler full of sherry.
He said it was good for me, and I'm sure it was, but until then I certainly hadn't had sherry by the tumblerful. Going from Australia to South Africa wasn't all that pleasant. In fact, it was really ghastly: Going from South Africa to London, with a lot of young students who were returning to their colleges in England, turned out to be delightful.
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The trip from Liverpool to Boston, on the other hand, was unbelievable. The Jews were fleeing Europe in great numbers, and the ship was absolutely crammed. We ran into huge storms in the North Atlantic — it couldn't have been a worse four days. I arrived only a few days after the hurricane, a truly devastating one, had gone up the coast of New England, flattening Boston.
Cleve had a delightful little house in the suburb of Jamaica Plain, where I think he said 35 trees were blown down in the yard. He met me at the ship and I stayed the first night in his house. This was the city where Edison first used electricity, yet as Cleve said, he had to show me to my room with a candle. It was not a good introduction to Boston. But he then arranged for my accommodation, and I worked very closely with him for the following three years.
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For the first year I was his personal assistant and spent a huge amount of time staining the slides which we had made in Australia. I tried a technique which hadn't ever been tried before — one of the silver stains which are used for Bodian's technique — and it worked remarkably, showing up a lot of structures which we hadn't seen very clearly before. But although he was very generous, financially it was a fairly difficult time. The second year I was there, he arranged for me to get a Lehman Fellowship.
Well, I did courses at Harvard — and I was immediately enrolled as a postgraduate student, which I hadn't originally thought of but which they said was essential. That meant I could take courses 'for credit' and also participate in their excellent system of 'auditing' courses. That is to say, if there was a perceived gap in your knowledge, you were told to sit in on a course in invertebrate palaeontology or invertebrate anatomy or whatever it was, without having to take the exams, to fill the gap.
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That was a very good way of learning. I didn't have the pressure of the examination to worry about, yet I learned from marvellous people like Alfred Sherwood Romer, on invertebrate structures, and others in so many fields — things I'd never dreamed about from Sydney which were then appearing as major topics. In endocrinology, progesterone had just been purified, one of the few hormones understood at that stage.
That was a hugely developing field. Cytology was tremendously exciting. The plant cytologists, in particular, had just learned how to break chromosomes with X-rays, neutrons, gamma rays, and so I was with the little group of students who were into that area during such extraordinary opportunities as when Sturtevant came into the group for a year as a visiting professor.
And I had come from Australia, where facilities were poor, into a lab where everything was provided, the fitting-out was luxurious.
There was a lot more money around than we would ever have conceived of. We had virtually no money to run the department. After his work on termite protozoa he got into cytology in a big way, and did a great deal on the structure of chromosome. He had the best conceivable Zeiss microscopes. In fact, if Zeiss came out with something new, the agent would come and ask Cleveland to test it, and provide him with all the best equipment — polarising equipment, for example.
So we were able to do as well as anybody. Tall, gangling in stature, a hardworking, delightful, generous person.
The family was extremely good to me in many ways — taking me with them one year on a vacation up to Canada. There was his wife Dorothy, who came to Australia and is now 88, their son, and Cleve's daughter Elaine, from a previous marriage, whom he worshipped. I remember her as a gorgeous little girl with beautiful golden curls. She was obviously highly intelligent and married a doctor in Boston, but her death in her 20s had a devastating effect on both Dorothy and Cleve. I couldn't have asked for a better mentor. I had started work on insect endocrinology, which was then an important topic, but when I was about halfway into a thesis, a Swede named Hanstrom brought out a whole book doing nearly everything I had done and planned to do.
I was pretty devastated — a limited time ahead and now, in , the European war had started and I had no way of getting back to Australia. The Pacific war was not on at this stage, but nevertheless travel was restricted for civilians. Thinking I couldn't go any further, I did a Masters degree and then with this fellowship was able to stay on and complete a doctorate.
But because Hanstrom had published his book I just didn't know what I was going to do until Cleveland said, 'Well, you've done all the Stolotermes material' — that was the stuff that he, and then we, had collected in the east coast and that I had collected in South Africa — 'Write it up. And it was a complete change.
That was a marvellous three years. I was in an extraordinary group of students. You might say that we all worked like slaves: I think we learned as much from each other in this very active group as we did from our professors, because we were all at the cutting edge and could benefit by our interactions. On getting your doctorate and finishing the work that Cleveland had so generously set up for you, you were invited to go down to St Louis. At a growth symposium — these were high-powered small groups of people who got together — I had met F O Schmidt, the professor in charge of the Department of Zoology at Washington University in St Louis, and told him that in studying the marvellous termite protozoa we had seen that the mytotic spindle was birefringent.
Schmidt had been struggling with the material at his disposal to try and find some birefringents in this mytotic spindle; now we had shown it clearly. So immediately after the meeting he came down to our lab and saw these things. The upshot was that because he was going to head up a department at MIT and his position in cytology at Washington University would be vacant, I was appointed as a junior lecturer there.
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I was at Washington University for one year, during which I was in the lab one Sunday afternoon, listening to a symphony concert on the radio, when President Roosevelt interrupted to say that Pearl Harbor had been bombed. This was at the time it was actually happening! And so the sky fell in for everybody. I think nobody will forget it. I was teaching medical students — mainly pre-meds, as they were at that stage — and all these kids wanted to go immediately and join up: We're going to do our bit. Don't go and join the army right now just because this has happened.
The country's going to need doctors. That's where your job is. Well, very soon a young man with whom I had been at prep school rang me. He had been doing business administration at Harvard but had immediately gone to Washington, and now told me I had to go there too. So within a fortnight I had packed up and left Washington University to go to Australian War Supplies Procurement, an organisation which was charged with doing with all the purchasing for Australia, for the defence forces, for health, everything.