But all evolutionary biologists know that variation itself is nature's only irreducible essence. Variation is the hard reality, not a set of imperfec… - Stephen Jay Gould

Life is a ramifying bush with millions of branches, not a ladder. Darwinism is a theory of local adaptation to changing environments, not a tale of inevitable progress. “After long reflection,” Darwin wrote, “I cannot avoid the conviction that no innate tendency to progressive development exists.”
Jastrow might argue that he is only considering the single pathway through the immense labyrinth of life’s bush that happened to lead to us. Even here I might reply that while we have a personal motive for special interest in (and affection for) this particular pathway, we have no right to regard it (or any other) as the essential direction of life. The pathways leading to aardvarks, anchovies, or artichokes are just as long, intricate, and biologically informative.

But our strong desire to identify trends often leads us to detect a directionality that doesn’t exist, or to infer causes that cannot be sustained. The subject of trends has inspired and illustrated some of the classic fallacies in human reasoning. Most prominently, since people seem to be so bad at thinking about probability and so prone to read pattern into sequence of events, we often commit the fallacy of spotting a “sure” trend in speculating about causes, when we observe no more than a random string of happenings.

If Ediacara survivors had been able to evolve internal complexity later on, then the pathways from this radically different starting point would have produced a world worthy of science fiction at its best.