Fossil Evolutionary Intermediates

Creationists and evolutionists have disagreed over intermediate fossils for decades. An intermediate fossil is one that seems to be an evolutionary transition between two groups of organisms. If all life was the result of evolution, there must have been innumerable intermediates that existed, and many of these should still exist as fossils. If this were true, and if we could view a video of the history of life we would see a continuous series of life forms, gradually changing from one type to another.

According to the biblical description of creation, within a week all the major types of life were created. Then perhaps evolutionary changes (microevolution and speciation) occurred within each of these created groups, but major groups, like worms, crabs, reptiles, birds, and humans did not evolve from common ancestors, and evolutionary intermediates between the major types never existed. Notice that in this creation concept there are two categories of evolution: some evolution (adaptation) did occur, at least within groups of species. However, larger scale evolution, e.g. evolution of reptiles from amphibians or pterosaurs from other reptiles, did not occur. Which of these two options does the evidence support? I won’t try to answer this in detail (this is a blog, not a book), but will consider some reasons why I think the fossil record leaves considerable room for debate over the nature of the fossil record of presumed intermediates.

First of all, there is abundant reason why creationists do accept microevolution and adaptation of organisms to changing environments as very real processes. It would be no surprise if fossil intermediates at this level were common. The odd thing is that they are not numerous as fossils. In the fossil record the most common picture is that one fossil species disappears from the record and another appears, without evidence of evolution from one to the other.

The bigger issues arise when we consider intermediates between the higher categories, such as orders, classes, and phyla (loosely described as body plans) of organisms. There is generally a lack of fossil intermediates between most of these groups. But there are a few cases in which there are fossils described in the literature as intermediates. This is where most of the controversy arises. The most prominent of these presumed intermediates are between fish and amphibians, amphibians and reptiles, therapsid reptiles and mammals, dinosaurs and birds, and between terrestrial mammals and whales. I will suggest reasons why it may be difficult to determine with any finality whether these really are transitional forms or not. The first reason is the evidence that the earth once had a much wider variety of creatures than those that survived to the present day. In addition to those groups that are extinct, other groups have many fossil forms but few living representatives. The Creator was not stingy with variety of life forms. This may be a reason why some presumed intermediates, like Archeopteryx and others may not be evolutionary intermediates, but simply evidence of a greater diversity of original organisms.

For some groups there is no discussion of intermediate forms, because there are none. The first fossil bats and pterosaurs (flying reptiles) are fully developed flying animals, and there are no intermediates. Perhaps this is because flight requires a whole suite of integrated structures, so they will either be fully flying or not flying at all. Thus it is all or nothing - there were no forms that were partly flying and thus no intermediate fossils. Other body forms, like amphibians and reptiles are not radically different in their structural requirements, and there can be a variety of types, not so different from each other.

In other cases there are fossils that seemed to be good intermediates, but new fossils changed the picture. For example there are a number of forms considered to be transitional forms between fish and amphibians. Then good fossil amphibian tracks were found lower in the geologic record, dating by radiometric methods at about ten million years older than any of the intermediates. Whether or not it is assumed that the radiometric dates are valid, this leaves the interpretation of the intermediate fossils in question.

One more thing I have observed is that creationists and non-creationists emphasize the observations that fit their views best. If a few fossils can be interpreted as evolutionary intermediates the creationists emphasize how little evidence there is for intermediates. Meanwhile the non-creationists emphasize the presence of these intermediates and how important they are. Neither of these conclusions is significant unless there is some type of quantitative evaluation of the abundance of intermediates.

The fossil record of presumed intermediates has puzzles and unanswered questions for everyone. They don’t provide good reasons to change one’s views of the history of life. The best approach is to wait and see what new evidence will turn up in time.


Leonard Brand

Loma Linda University

April 2, 2014