In addition to steps that are hard in the sense that they can take a long time, there may also be tricky steps—ones that have a finite window of opportunity before being precluded by some other version of events. I don’t have a good enough theory of the development of civilization to defend any candidates in history, but here’s a paleontological candidate:
Consider that animal lineages readily lose limbs but, once lost, almost never regain them (many-legged arthropods to six-legged insects, four-legged reptiles to two-legged birds and no-legged snakes, but no reverse transitions; the reasons for this are easy to understand in terms of the requirement of evolution that intermediate forms be advantageous). It is also clear that animals can get by well enough with two legs, but sparing two of four for toolmaking has always been a problem, let alone sparing two of two. Finally, it is clear that four legs over two is not necessarily enough advantage to displace an entrenched competitor.
The four-legged lungfish that became the ancestor of amphibians could not of course have known its distant descendents would need two spare limbs for toolmaking. If it had been delayed until a two-legged lungfish had taken the niche… it is not certain that the ultimate development of civilization would have been rendered impossible, but it is at least plausible.
In addition to steps that are hard in the sense that they can take a long time, there may also be tricky steps—ones that have a finite window of opportunity before being precluded by some other version of events. I don’t have a good enough theory of the development of civilization to defend any candidates in history, but here’s a paleontological candidate:
Consider that animal lineages readily lose limbs but, once lost, almost never regain them (many-legged arthropods to six-legged insects, four-legged reptiles to two-legged birds and no-legged snakes, but no reverse transitions; the reasons for this are easy to understand in terms of the requirement of evolution that intermediate forms be advantageous). It is also clear that animals can get by well enough with two legs, but sparing two of four for toolmaking has always been a problem, let alone sparing two of two. Finally, it is clear that four legs over two is not necessarily enough advantage to displace an entrenched competitor.
The four-legged lungfish that became the ancestor of amphibians could not of course have known its distant descendents would need two spare limbs for toolmaking. If it had been delayed until a two-legged lungfish had taken the niche… it is not certain that the ultimate development of civilization would have been rendered impossible, but it is at least plausible.