Despite the overwhelming evidence for macroevolution, part of my mind still battles to accept the fact that such large changes can occur in living organisms. Why is this so? Why do some, if not many, of us have a hard time accepting and understanding biological evolution?
Richard Dawkins, in the preface of his book The Blind Watchmaker, provides possible reasons. First, humans are successful creative designers. So much so that we are predisposed to the idea that complex elegance is an indication of purposeful design. Evolution is counter-intuitive in this sense. Second, our brains are built to deal with events on totally different timescales from those that characterise evolution. We can grasp timescales of days, months, years or decades, but battle to understand change that takes place over millions of years.
This last point is important when one is thinking about the differences between microevolution and macroevolution. Microevolution is easy to grasp because it takes place over a period of years and decades. However, our minds naturally stumble when it comes to macroevolution. How do we overcome this problem? The key to understand macroevolution is to realise that if you give microevolution enough time, it will result in macroevolutionary change. In other words, macroevolution is simply a whole lot of microevolution.
Take the human embryo, for example. Straight after conception, hundreds of cellular and genetic changes - all tiny changes - take place over a period of nine months to transform the little pack of cells into a human baby. But this is not all: thousands of changes occur to transform the baby into a child, then into a teenager, then into an adult - over a period of many years. If someone compared a photo of an embryo to an adult human, they might – hypothetically speaking - find it unbelievable that something so small can change into something so large and so different. A similar analogy can be drawn regarding an oak tree: how can such a small acorn develop into a massive, complex organism? Such macro-change does seem unbelievable, but it does happen, thanks to all the small micro-changes that take place over long periods of time. We can see this kind of thing happening in the natural world all around us.
Evolution is similar: it is difficult to imagine how birds evolved from reptiles, or how humans evolved from small mammalian-like creatures. Our brains battle to conceptualise large changes. But if we allow thousands upon thousands of natural micro-changes over millions of years, it doesn't seem that impossible at all.