I understand that this is what is assumed, but there is no evidence to support the assumption.
To claim the mechanism that gets us variety among equines, canines, finches, fruit flies, etc. can get us from bacteria to all the various lifeforms that have existed is pure philosophical assumption. To play off of your analogy, it's like saying since you can walk from California to Maine you can walk from Earth to Mars if you're just given enough time. It is never going to happen naturally.
A man can, by taking one step at a time, walk from Maine to California. He cannot walk from California to Hawaii, or as you said, to Mars. but that is not due to any limitation in how far he can walk. He can walk from Maine to California and back again, a distance longer that that from California to Hawaii, and over his lifetime he can even walk the distance from Earth to Mars. The limitation is not the distance, but conditions unrelated to distance.
Have you ever seen a child digging a hole to China? You know that he'll never make it. He'll probably be called in for dinner before he gets a few feet, and over the next few days, his parents will either fill in the hole or make him fill it in. But even if his parents allow him to continue he will be stopped when he reaches either the water table (turning his hole into a well) or bedrock (which is too hard for his shovel to penetrate).
As adults, we have ways of digging holes and tunnels past these obstacles and other ones like collapsing tunnel walls, etc. This is analogous to our building boats to get to Hawaii. Nature can build "boats," too, and make huge leaps.
So then, is building a tunnel to china is just a matter of angling it right ang taking the necessary time? No because there are new challenges when we reach the Mantle and Core layers. These are like the challenges of passing up into and through the various layers of the atmosphere on our way to Mars -- or maybe more like the challenge of escaping the earth's gravity. But we have overcome those challenges, too.
But back to the child. He knows nothing about these challenges. His experiences tel him that digging one foot down, the dirt is almost the same as when he started and digging in the same way at the bottom of the one foot hole takes him another foot down, to more dirt that is still almost the same.
If you told him about the difficulties he might not believe you, and if he did believe you he might still continue digging, but to a different purpose. He would now be focused on seeing those difficulties for himself.
If instead of telling him about the difficulties, you simply told him it was impossible, he might believe you, and might never dig another hole, and tell anyone else he saw digging holes that it was a waste of time, but more likely he would not believe you until he found those difficulties for himself. If he was stubborn, he would still be in a mood to defy your fiat of "impossible," and find ways past the obstacles.
You are in the position of telling us that it is impossible. Worse, you are not doing so based on the difficulties, but because someone else told you it was impossible and you believed him, so when challenged, you have nothing to offer to back up your claim. Is it any wonder that the stubborn kids are stil digging, and that some of them have found some of the difficulties, and the ways around them? Yes there may be bigger difficulties ahead, perhaps even one that can't be gotten around. But we have no evidence what that "prime" difficulty could be, and even less that it is absolute.
It has no naturalistic explanation now, and my money is on it never will.
There
is a rationalistic explanation now ("The dirt two feet down is almost the same as the dirt one foot down, which is almost the same as the dirt at the top, so its reasonable to assume that the dirt three feet down is the same."). When we get three feet down we will know if it is true or not, but it is reasonable now.
