Reasons to Believe

Tetrapod Transitions: Evidence for Design

The ubiquitous bumper sticker featuring a fish sprouting legs has symbolized a sometimes rancorous contemporary debate: Does evolution or creation better explain life’s history? Recent fossil finds have put literal and figurative legs on the controversy by challenging Charles Darwin’s paradigm-shifting theory.

When Darwin advanced his theory of evolution he knew the fossil record didn’t support his idea. In The Origin of Species Darwin devoted a chapter to “difficulties” for his theory, with most of his focus on the fossil record. He was troubled by two of its features: the absence of transitional forms and the abrupt, first-time appearances of biological groups.

Given a Darwinian mechanism, which views evolution as proceeding in small, successive steps, the fossil record should display gradual transformations replete with corresponding intermediate forms. Yet Darwin acknowledged in Origin of Species that this wasn’t the case. He bemoaned, “But, as by this theory innumerable transitional forms must have existed, why do we not find them embedded in countless numbers in the crust of the earth?”

Darwin skirted this problem by arguing that the fossil record was incomplete and poorly studied. He expected that as paleontologists continued to collect fossils, eventually the missing transitional forms and gradual evolutionary transformations would be unearthed. Over the last 150 years, paleontologists have indeed uncovered a cache of fossils that reveal a rich history of life on Earth. Yet, precious few can be considered authentic transitional forms.

This near absence accounts in part for the excitement that’s been generated in the last few years by the discovery of a series of fossils that seem to document the evolutionary transition from lobe-finned fish to amphibians, the first animals on land. These “fishapods” possess characteristics that have been interpreted as varying mixtures of fish and amphibian features. Paleontologists regard Tiktaalik, the first of the proposed intermediates that made a splash when discovered, as the midway point between lobe-finned fish and amphibians.1

At first glance, Tiktaalik seemed to delineate a key evolutionary transition, thereby providing a strictly naturalistic explanation for the emergence of life on land. But recent discoveries of other “fish-amphibian intermediates” in Latvia raise questions about the evolutionary interpretation.

Paleontologists have analyzed the fairly complete remains of a creature called Ventastega.2 This animal lived about 365 million years ago and is thought to occupy a halfway point between Tiktaalik and amphibians. However, its skeletal features indicate that it’s out of sequence. Older fishapods actually exhibit more advanced features than those of Ventastega.

This same problem has recently been noted for another fishapod, Panderichthys.3 This creature existed about 385 million years ago and is considered to be much closer to a lobe-finned fish than an amphibian. Yet, it has digits at the end of its fins, whereas Tiktaalik, considered to be more advanced, doesn’t. Again, the fossils are out of sequence.

Another difficulty is the speed with which the transition from fish to amphibian appears to occur. According to the fossil record, the putative transition begins about 385 million years ago and finishes about 365 million years ago. Intuitively, this timescale seems too rapid considering the extent of the anatomical and physiological changes needed to transform an aquatic creature to one that lives on land.

Yet another concern for the evolutionary account is that the fishapods co-occur in the fossil record. Instead of appearing in a sequential fashion, the creatures evidently coexisted and overlapped. In other words, the pattern observed in the fossil record doesn’t describe a linear evolutionary change over time. The challenge for evolutionary biologists lies in letting the fossil record dictate the pattern rather than imposing a pattern on the data.

How does the existence of fishapods fit a creation interpretation of life’s history? For one, these animals were perfectly suited to live at the water’s edge. Also, the mosaic of fish and tetrapod characteristics possessed by the fishapods signifies design. Human designers frequently design objects and systems that blend properties and features of different systems.4 If humans create mosaics, why wouldn’t a Creator do the same?

These fascinating creatures, well-designed to exploit their environment, make sense from a creation perspective. Conversely, overlapping fishapods that appear in the “wrong” order in the fossil record indicate Darwin’s long-sought “innumerable transitional forms” still don’t exist.

References:

1 For a recent analysis of Tiktaalik see, Jason P. Downs et al., “The Cranial Endoskeleton of Tiktaalik roseae,” Nature 455 (2008): 925–929.

2 Per E. Ahlberg et al., “Ventastega curonica and the Origin of Tetrapod Morphology,” Nature 453 (2008): 1199–1204.

3 Catherine A. Boisvert, Elga Mark-Kurik, and Per E. Ahlberg, “The Pectoral Fin of Panderichthys and the Origin of Digits,” Nature (2008): advanced on-line, doi:10.1038.

4 One prosaic example of a mosaic is a “spork.” Some fast-food restaurants provide this eating utensil, which is a combination of a fork and a spoon.

Related resources that may be of interest:

10 Breakthroughs of 2010 booklet

"Out of Order" web article

"4-Legged Animals Emerged Earlier than Thought" Science News Flash podcast

 

Subjects: Fish-to-Amphibians Transitions

Dr. Fazale Rana

In 1999, I left my position in R&D at a Fortune 500 company to join Reasons to Believe because I felt the most important thing I could do as a scientist is to communicate to skeptics and believers alike the powerful scientific evidence—evidence that is being uncovered day after day—for God’s existence and the reliability of Scripture. Read more about Dr. Fazale Rana