If your goal is to make cross-species hybrids, there is this problem to contend with:
NEW YORK (AP) It may be futile to try producing stem cells by putting human DNA into cow or rabbit eggs and making hybrid cloned embryos, a strategy that triggered controversy recently in Britain, a new study says. The animal eggs don't reprogram human DNA in the right way to generate stem cells, researchers report.
"Instead of turning on the right genes, it turns out the animal eggs actually turn them off," said senior study author Dr. Robert Lanza of Advanced Cell Technology in Worcester, Mass.
Cows and rabbits are pretty far off – 140 million years or so of evolutionary separation might well have changed gene regulation in early embryogenesis so that the chemical signals in a donor egg wouldn’t work with a human genome. Other hominoids would be less likely to generate problems. Of course, egg cells from rabbits are easier to come by than egg cells from gorillas.
I’m more interested in the far-off possibility of cloning an extinct hominid. Would a Neandertal genome in a human egg lead to similar incompatibilities?
There are a few lines of argument against it. For one thing, the combination of paternal and maternal genetic material in fertilized eggs doesn’t tend to generate such incompatibilities between closely related mammals. The very fact that we see interspecific hybrids is a clue that the mechanisms of early embryogenesis are conservative, with few changes that would interrupt development.
A complementary argument is that different groups of people living today are distantly related enough to have seen some such incompatibilities, if they were likely to happen. People in the world today are, on average, more alike than any living human and a Neandertal. But the total range of mutational variation today exceeds the average human-Neandertal difference for any given gene.
On the other hand, there are reproductive incompatibilities today between some individuals within populations. And we don’t know whether interfertility is identical between every pair of populations. We often assume this, but who has carried out the experiment?
In any event, some kinds of genetic transfer will not work without being able to design custom cells. An egg cell that is not just taken out of a donor species, but is built to contain a specific array of signals, might avoid the problems with regulatory incompatibilities. Or it would give entirely new opportunities for epigenetic modification of genetic information.