Does evolution occur gradually or in “punctuated equilibrium”? Charles Darwin, a gradualist, maintained that life forms arise from others in a gradual and orderly fashion. Small modifications that accumulate over the generations add up to major changes after millions of years. Gradualists cite intermediate fossils as evidence for their position, concluding that there would be even more transitional forms if it weren't for gaps in the fossil record. The advocates of the punctuated equilibrium model believe that long periods of equilibrium, during which species change little, are interrupted (punctuated) by sudden changes―evolutionary leaps. One reason for such jumps in the fossil record may be extinction followed by invasion by a closely related species.
For example, a sea species may die out when a shallow body of water dries up, while a closely related species will survive in deeper waters. Then, later, when the sea reinvades the first locale, the protected species will extend its range to the first area. Another possibility is that when barriers are removed, a group may replace, rather than succeed, related one because it has a trait that makes it adaptively superior in the environment they now share. When a major environmental change occurs suddenly, one possibility is for the pace of evolution to increase. Another possibility is extinction. The earth has witnessed several mass extinctions―worldwide ecosystem catastrophes that affect multiple species. The biggest one divided the era of “ancient life” (the Paleozoic) from the era of “middle life” (the Mesozoic). This mass extinction occurred 243 million years ago, when 4.5 mil-lion of the earth's estimated 5 million species (mostly invertebrates) were wiped out. The second biggest extinction, which occurred 65 million years ago, destroyed the dinosaurs and many other Mesozoic species. One explanation for the extinction of the dinosaurs is that a massive, long-lasting cloud of gas and dust arose from the impact of a huge meteorite. The cloud blocked solar radiation and therefore photosynthesis, ultimately destroying most plants and the chain of animals that fed on them. The hominid fossil record exemplifies both gradual and rapid change, confirming that evolution can be faster or slower depending on the rate of environmental change, the speed with which geographic barriers rise or fall, and the value of the group's adaptive response. Australopithecine teeth and skulls show some gradual transitions. For example, some of the fossils that are intermediate between Australopithecus and early Homo combine a larger brain (characteristic of Homo) with huge back teeth and supportive structures (characteristic of the australopithecines). However, there is no doubt that the pace of hominid evolution sped up around 18 million years ago. This spurt resulted in the emergence (in just 200,000 years) of Homo erectus. This was followed by a long period of relative stability. The probable key to the rapid emergence of Homo erectus was a dramatic change in adaptive strategy: greater reliance on hunting through improved tools and other cultural means of adaptation. The new economy, tools, and phenotype arose and spread rapidly, then remained fairly stable for about 1 million years.