Imagine that you create a very large cage and put a group of mice into it. You let the mice live and breed in this cage freely, without disturbance. If you were to come back after five years and look into this cage, you would find mice. Five years of breeding would cause no change in the mice in that cage -- they would not evolve in any noticeable way. You could leave the cage alone for a hundred years and look in again and what you would find in the cage is mice. After several hundred years, you would look into the cage and find not 15 new species, but mice.
The point is that evolution in general is an extremely slow process. When two mice breed, the offspring is a mouse. When that offspring breeds, its offspring is a mouse. When that offspring breeds... And the process continues. Point mutations do not change this fact in any significant way over the short haul.
Carl Sagan, in "The Dragons of Eden," put it this way:
The fact that it takes evolution 100,000 or 10 million years to make relatively minor changes in existing structures shows just how slow evolution really is. The creation of a new species is time consuming.
On the other hand, we know that evolution can move extremely quickly to create a new species. One example of the speed of evolution involves the progress mammals have made. You have probably heard that, about 65 million years ago, all of the dinosaurs died out quite suddenly. One theory for this massive extinction is an asteroid strike. For dinosaurs, the day of the asteroid strike was a bad one, but for mammals it was a good day. The disappearance of the dinosaurs cleared the playing field of most predators. Mammals began to thrive and differentiate.
Example: The Evolution of Mammals
65 million years ago, mammals were much simpler than they are today. A representative mammal of the time was the species Didelphodon, a smallish, four-legged creature similar to today's opossum.
In 65 million years, according to the theory of evolution, every mammal that we see today (over 4,000 species) evolved from small, four-legged creatures like Didelphodon. Through random mutations and natural selection, evolution has produced mammals of striking diversity from that humble starting point:
Evolution has created thousands of different species that range in size and shape from a small brown bat that weighs a few grams to a blue whale that is nearly 100 feet (30.5 m) long.
Let's take Carl Sagan's statement that "A characteristic period for the emergence of one advanced species from another is perhaps a hundred thousand years, and very often the difference in behavior between closely related species -- say, lions and tigers -- does not seem very great." In 65 million years, there are only 650 periods of 100,000 years -- that's 650 "ticks" of the evolutionary clock.
Imagine trying to start with an opossum and get to an elephant in 650 increments or less, even if every increment were perfect. An elephant's brain is hundreds of times bigger than an opossum's, containing hundreds of times more neurons, all perfectly wired. An elephant's trunk is a perfectly formed prehensile appendage containing 150,000 muscle elements (reference). Starting with a snout like that of an opossum, evolution used random mutations to design the elephant's snout in only 650 ticks. Imagine trying to get from an opossum to a brown bat in 650 increments. Or from an opossum to a whale. Whales have no pelvis, have flukes, have very weird skulls (especially the sperm whale), have blow holes up top, have temperature control that allows them to swim in arctic waters and they consume salt water rather than fresh. It is difficult for many people to imagine that sort of speed given the current theory.
Example: The Evolution of the Human Brain
Here is another example of the speed problem. Current fossil evidence indicates that modern humans evolved from a species called Homo erectus. Homo erectus appeared about 2 million years ago. Looking at the skull of Homo erectus, we know that its brain size was on the order of 800 or 900 cubic centimeters (CCs).
Modern human brain size averages about 1,500 CCs or so. In other words, in about 2 million years, evolution roughly doubled the size of the Homo erectus brain to create the human brain that we have today. Our brains contain approximately 100 billion neurons today, so in 2 million years, evolution added 50 billion neurons to the Homo erectus brain (while at the same time redesigning the skull to accommodate all of those neurons and redesigning the female pelvis to let the larger skull through during birth, etc.).
Let's assume that Homo erectus was able to reproduce every 10 years. That means that, in 2 million years, there were 200,000 generations of Homo erectus possible. There are four possible explanations for where the 50 billion new neurons came from in 200,000 generations:
- Every generation, 250,000 new neurons were added to the Homo erectus brain (250,000 * 200,000 = 50 billion).
- Every 100,000 years, 2.5 billion new neurons were added to the Homo erectus brain (2,500,000,000 * 20 = 50 billion).
- Perhaps 500,000 years ago, there was a spurt of 20 or so closely-spaced generations that added 2.5 billion neurons per generation.
- One day, spontaneously, 50 billion new neurons were added to the Homo erectus brain to create the Homo sapiens brain.
None of these scenarios is particularly comfortable. We see no evidence that evolution is randomly adding 250,000 neurons to each child born today, so that explanation is hard to swallow. The thought of adding a large package of something like 2.5 billion neurons in one step is difficult to imagine, because there is no way to explain how the neurons would wire themselves in. What sort of point mutation would occur in a DNA molecule that would suddenly create billions of new neurons and wire them correctly?* The current theory of evolution does not predict how this could happen.
One line of current research is looking at the effect of very small changes in DNA patterns during embryonic development. Any new animal, be it a mouse or a human, starts life as a single cell. That cell differentiates and develops into the complete animal. A tremendous amount of signaling happens between cells during the development process to ensure that everything ends up in the right place. Tiny changes in these signaling processes can have very large effects on the resulting animal. This is how the human genome, with at most 60,000 or so genes, is able to specify the creation of a human body containing trillions of cells, billions of carefully wired neurons and hundreds of different cell types all brilliantly sculpted into organs as diverse as the heart and the eyes. The book "Molecular Biology of the Cell" puts it this way:
Humans, as a genus distinct from the great apes, have existed for only a few million years. Each human gene has therefore had the chance to accumulate relatively few nucleotide changes since our inception, and most of these have been eliminated by natural selection. A comparison of humans and monkeys, for example, shows that their cytochrome-c molecules differ in about 1 percent and their hemoglobins in about 4 percent of their amino acid positions. Clearly, a great deal of our genetic heritage must have been formed long before Homo sapiens appeared, during the evolution of mammals (which started about 300 million years ago) and even earlier. Because the proteins of mammals as different as whales and humans are very similar, the evolutionary changes that have produced such striking morphological differences must involve relatively few changes in molecules from which we are made. Instead, it is thought that the morphological differences arise from differences in the temporal and spatial pattern of gene expression during embryonic development, which then determine the size, shape and other characteristics of the adult.
In other words, there just are not that many differences in the DNA of a human and a whale, yet humans and whales look totally different. Small collections of DNA mutations can have a very big effect on the final result.
Right now, the signaling mechanisms that wire up the 100 billion cells in the human brain are something of a mystery. How can the mere 60,000 genes in the human genome tell 100 billion neurons how to precisely wire themselves in the human brain? No one right now has a clear understanding of how so few genes can meticulously wire so many neurons. In a developing fetus in the womb, DNA is correctly creating and wiring up millions of cells per minute. Given that DNA does wire up a working human brain every time a baby is born, it may be the case that DNA has special properties that make evolution work more efficiently. As the mechanisms become better understood, the effects of DNA mutations during development will become better understood as well.