Don't just laugh at the advances in genetic science. It took about four decades of research to get to Dolly. That was 28 years ago. Dolly wasn't the first mammal to be cloned. They had another sheep in 1984, 40 years ago to current year. Around 50 years ago no one would have imagined that this is possible and like you would have requested for the source.

What made Dolly so special was because she had been made from an adult cell, which no-one at the time thought was possible. Dolly was the end product of what's called somatic cell nuclear transfer.
This involves removing the genetic material from an egg and replace it with the nucleus of some other body cell. The resulting egg becomes a factory to produce an embryo that develops into an offspring. No sperm is in the picture; instead of half the genetic material coming from a sperm and half from an egg,
it all comes from a single cell. It’s diploid from the start.
Genetics have come a long way since Dolly and resurrecting an extinct species is not exactly just a pipe dream.