"Thousands of years ago, humans domesticated every possible large wild mammal species fulfilling all those criteria and worth domesticating, with the result that there have been no valuable additions of domestic animals in recent times, despite the efforts of modern science"

This quote by Jared Diamond recommends that human beings have actually succeeded in domesticating all the large wild mammal types that deserved domesticating thousands of years back. This implies that despite the efforts of modern science, there have been no new important additions of domestic animals in current times. This might be due to the fact that all the types that were worth domesticating have currently been domesticated, leaving no space for brand-new additions. This quote highlights the success of people in the past in domesticating animals, and the absence of success in current times.