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IT MAY seem bizarre, but when Dolly was born I was gardening. It was July 1996, and we knew that, if this lamb survived, she would be the first animal ever cloned from an adult cell. I had issued instructions that only people who needed to attend the birth should be there. I followed my own rule.

My colleague Keith Campbell, who had such a big role, was on holiday. When he came back everybody went for dinner. The media circus came much later 鈥� in February of the following year, when .

With a distance of 20 years, it may seem simple to retell it in a clear, step-by-step timeline. But it鈥檚 more jumbled than that. Dolly changed a fundamental aspect of how we understand biological development, but her arrival didn鈥檛 neatly splice the world into everything that came before and the new dawn that came after.

I first came to what would in Edinburgh after doing research in reproductive biology at the University of Cambridge. There I had developed ways of freezing cow embryos, and that led to the birth of , in 1973.

A few years after I came to Roslin, around 1980, the research priority became molecular genetics, including genetic modification. We hoped to be able to do 鈥済ene targeting鈥�, to introduce genetic changes in animals that would result in the production of selected proteins, such as antibodies, that could be used for therapeutic purposes. The idea was to take zygotes 鈥� very recently fertilised eggs 鈥� and incorporate human DNA into them.

By injecting a few hundred copies of a human gene into a zygote, we were soon able to produce sheep that generated human proteins in their milk. But only 1 per cent of the zygotes inherited the gene, and we could only add genes 鈥� not remove or modify existing ones. It wasn鈥檛 terribly efficient. So we were looking for better ways to do it.

In 1981, a paper had been published showing that mice could be cloned by nuclear transfer, which meant removing the nucleus from an unfertilised egg and replacing it with the nucleus of an early embryonic cell. This got our attention. But efforts to replicate that work came to nothing. In fact, in 1984 two highly regarded researchers went so far as to .

But then, in 1986, a researcher called Steen Willadsen managed . And in a bar room conversation a year later, I learned that he had also produced live calves using this method. I thought, Bloody hell!

Embryonic cells appeared to accept genetic modification more readily than most other cell types, so I thought, if this report is true, nuclear transfer would be a really useful technique for genetic modification. It was a real light bulb moment.

So I visited Willadsen, who was extremely generous in describing his research. And I returned to Edinburgh with a plan to move towards cloning by nuclear transfer.

A project this big needed a great team. A key player we recruited was Keith. He had done work on the cell nucleus and had probably thought about cloning for longer than most.

Over the next few years we worked out how to clone lambs from embryonic cells. Then we moved on to cells taken from later-stage embryos. At the time, the understanding was that once cells had begun to differentiate 鈥� specialise into skin cells, for instance 鈥� then they lost or shut down the genetic instructions necessary to become anything else.

Yet Keith had long thought that at some point adult cells would be used for cloning. It wasn鈥檛 on my agenda. But now we were in a position to try. The year before Dolly, we 鈥� from embryonic cells that had begun to differentiate. So next we looked at whether we could use even later stage cells, from adults.

Each experiment demanded huge numbers of sheep to provide eggs and act as surrogate mothers, so we couldn鈥檛 do them all in one go. But then the company we were collaborating with, PPL Therapeutics, unexpectedly had some issues with cell lines. They had to stop work for a while, which meant they suddenly had a considerable number of sheep available. We sat down around Christmas 1995 and debated what to do with these ewes. That鈥檚 when we devised the Dolly experiment.

Using cells from adult mammary tissue already cultured and frozen by PPL, we did nuclear transfer into 277 eggs. Just 29 embryos were implanted into surrogate mothers. From those, only Dolly was born. We didn鈥檛 know for sure right up to the end.

It was a Sunday in February 1997 when the story broke. We were prepared for some attention but this was on another level. By Monday the car park was cluttered with television satellite trucks. One group wanted a photo using a mirror. So they went and got one and I stood in a field, getting very cold, so they could edit Dolly into the picture later.

We knew we had opened the door on something big, but we didn鈥檛 actually know what was going to be possible. It was exciting to see how powerful the technique might be for therapeutic cloning 鈥� to grow specific tissues for transplant, for instance. But it was also exciting because of how it changed our understanding of biology.

Of course, when the news broke the conversation turned to human cloning. Keith and I saw that coming. He sometimes gave me a lift home and we had talked about it on the journey. We knew human cloning might be possible, and that after Dolly, we鈥檇 be asked about it. We had agreed: we didn鈥檛 like the idea.

That aspect of the story more than any other kept the media storm going. I can remember saying to my wife, 鈥淥h don鈥檛 worry, it will all have died down by autumn鈥�. I was a fool.

鈥淔rom 277 eggs, only Dolly was born. We didn鈥檛 know for sure right to the end鈥�

Shortly after the paper was published, Keith left Roslin. For the next few years, both he and I gave huge numbers of talks and even . Dolly created many opportunities. But some people have clearly felt that these weren鈥檛 shared fairly 鈥� particularly between Keith and me. For the cloning research and my earlier work, I was nominated to be a fellow of the Royal Society, and later, given a knighthood. I learned how that rankled for some.

In that time, Keith鈥檚 life became more complicated, I believe he faced some difficult personal problems. Then, in 2012, he died unexpectedly. In conversations I had after his death, it became clear to me that few people knew about the sequence of events that led to my being the first author on the Dolly paper. But .

Keith was first author on all of the early work and me last. When we planned the next paper, the Megan and Morag experiment, we agreed that I would be first author because I had planned the experiments and advocated working toward nuclear transfer from cultured cells to begin with. With Megan and Morag, though, Keith introduced important new work, so I said he should be first author on that one as well, and I would have the next one. At the time, of course, none of us knew that the next one was going to be Dolly.

When I look back on what our big team achieved, I think we still don鈥檛 understand the biology behind it. Not really.

Perhaps I thought we would understand more by now, and for what it鈥檚 worth I think that we will one day. Stem cell research, which was built on the cloning experiments 鈥� built on the Dolly experiment 鈥� is starting to have an effect. There are now clinical trials under way. The promise of what Dolly represented is starting to be realised.

This article appeared in print under the headline 鈥淒olly & me鈥�