Evidence of Evolution

Evolution is descent with modification

Modification - Evolutionary Processes:

Natural Selection: Most educated people have learned about Charles Darwin's Theory of Evolution by Natural Selection, as least in high school, so we needn't belabor it here. Quick review of Darwin's tenets:

The 20th century New Synthesis - the marriage of evolution with the sciences of genetics and embryology made us aware of other mechanisms of evolutionary change, including:

Descent - Evolutionary Patterns:

Although all of this can be demonstrated in the laboratory, the fact that it is hard to see on a human time scale has limited the public's willingness to accept the reality of the phenomenon at all.

Perhaps the real demonstration of of evolution lies in recognizing the pattern that it generates in living and ancient creatures. For example:

A cladogram (stick-figure evolutionary tree) of the familiar land vertebrates. It shows:

Cladistics: The science of Systematics - the ordering of the diversity of life - employs repeatable "cladistic" algorithms to identify the most Parsimonious potential arrangements of the tree of evolution that minimize the number of evolutionary novelties necessary to account for their distribution. (The rise of cladistics was an intellectual revolution in systematics that occurred during the late 20th century. More on this in BSCI106.) Thus, cladograms are hypotheses of evolutionary history that can be tested by the addition of new data.
Sources of evidence: The physical evidence that informs these parsimony analyses mostly comes from three sources:


Evolutionary Time: Assessing the tempo of evolutionary change

First, some background on the chronology of fossils:

Rock Types:

Stratigraphy - Turning the sedimentary record into history: Sedimentary rocks have the advantage that their relative ages can be determined without complex technology.

Absolute dating:

So fossils tell us something about the age of rocks, but what do rocks tell us about the age of fossils? Prior to the 20th century researchers really didn't have any idea of the numeric ages of the rocks they studied. That changed with the discovery of:

Taken together, the evidence of the rock record enables us to establish the chronology of the history of life with reasonable fidelity.

Fossils and Paleontology: Fossils: Are any record of past life incorporated into the rock record. Paleontology is the study of the fossil record. We encounter fossils in sedimentary rocks because, unlike other rocks, sedimentary rocks form where life lives.

My purpose today is to provide examples of the reciprocal-illumination of these sources of data in the interpretation of some major evolutionary transformations in vertebrate history.

Compelling case studies I: Whales Learn to Swim

Evolutionary novelties of living whales:

Early artiodactyl Diacodexis from The Fossil Forum
These are remarkable adaptations, but all the more so when one considers the starting point for this evolutionary trend: a primitive even-toed ungulate like Diacodexis (right). Today, we think of these creatures as swift herbivores, but during the early Cenozoic Era (age of mammals), artiodactyls experimented with a variety of life styles including: How we got to this conclusion is an interesting story.

Eocene whale Dorudon atrox from The Lord Geekington
Fossil whales: Linnaeus (1759) identified whales as mammals. Fossil whales have been known for over a century. The earliest unambiguous fossil whales, like Dorudon (right) although clearly modified for marine life, retained obvious vestiges of their earlier lives as terrestrial animals:

During the early 1990s, paleontologists were very happy with their progress, as the evolutionary history of early whales was being filled in quite nicely by new discoveries, including the skull of Pakicetus, which seemed to be near the terrestrial aquatic transition. (Link to an early 90s style Pakicetus reconstruction.)

The mesonychid Mesonyx by Charles Knight colored by Jennarotancrede
The mesonychid hypothesis:

Mesonychids were impressive carnivorous hoofed mammals of the early Paleogene, and included Andrewsarchus, the largest known terrestrial mammalian predator. The anatomical similarity of the skulls of early fossil whales know in the late 20th century to those of mesonychids led paleontologists to assume that whales were ultimately derived from them. The similarity of their teeth was particularly convincing to paleomammologists who were accustomed to squeezing information out of the complex teeth of mammals. Mesonychids, themselves, were close to the ancestry of artiodactyls, but clearly outside of it. Therefore so were whales - the 1990s paleontological consensus.

The "whippo" hypothesis: During the mid-1990s, repeated molecular phylogenetic analyses yielded very robust support for a close relationship between whales and, specifically, hippopotamuses. To paleontologists, this seemed bizarre:

Paleontologists took to calling the molecular conclusion "the whippo hypothesis," but the result would not go away. Through it all, the discovery of relevant fossils continued, and cladists applied their trade to ever-expanding datasets. Just after the turn of the century, this new data caused reconsideration. First, a synopsis of what was learned:

Landmarks in cetacean evolution: Watch for the following trends:

The cetacean Pakicetus
Pakicetus expanded: The discovery of additional material made this among the best known primitive cetaceans. Shares with more derived cetaceans: Pakicetus's eye sockets sit close together on top of its skull, suggesting that it could watch potential prey or danger while mostly submerged. Not a feature of more derived whales, but reminiscent of hippos. Clearly, it could walk around on shore. (Link to a contemporary reconstruction.)

The cetacean Ambulocetus from Research Casting International
Ambulocetus: Roughly contemporaneous with Pakicetus (Eocene epoch - Cenozoic), Ambulocetus provides post-cranial remains that shed light on its locomotor adaptations. Ambulocetus remains are found in near-shore marine and estuarine deposits. The shape of its skull is reminiscent of a crocodylian, but its manner of axial swimming, involving both the vertebral column and the hindlimbs, is reminiscent of an otter. Perhaps it could also prey on land animals.

Rhodocetus: (Eocene epoch - Paleogene), Rhodocetus is also a limb and trunk propelled otter-style swimmer. It is slightly derived in the elongation of the trunk and the retraction of the nostrils compared to Ambulocetus.

Ankles of Rhodocetus and Antilocapra (pronghorn) in proximal view.
Of particular interest is its well-preserved ankle. This shows two items of note:

And this was the huge news. Whales are definitely some kind of artiodactyl. During the same interval, cladistic analyses were reexamining the relations of extant and fossil artiodactyls. The simple result when new data was included:

The Whippomorph Hypothesis has won the day by:

A classic case of reciprocal illumination.

Indohyus indirae from The Guardian.
What would their last common ancestor look like?

Hard to say as our record of ancient hippos is poor, but we do have this:

Compelling case studies II: The Origin of Bird Flight

Left: Allosaurus a "dinosaur" by Camus Altamirano Right: Malachite kingfisher, a "bird"

Bird origins and bird flight: The idea that birds are a kind of dinosaur that evolved the power of flight is not new. It was advocated in the late 19th century by "Darwin's bulldog," Thomas Henry Huxley, but fell into disfavor in the early 20th century. Starting in the 1964 with John Ostrom's description of the "raptor" Deinonychus, the possible dinosaurian origin of birds roared back to life as the center of a true academic psychodrama. By dumb luck, the issue had popped up just in time to become the poster-child for larger disputes about: The real problem is that when we limit our information to living creatures, a huge gulf separates birds from their closest relatives - crocodilians. It would take many fossil discoveries to close that gap.

The Berlin specimen of Archaeopteryx lithographica from Biologypop
Archaeopteryx - "The first 'bird'" (right - Late Jurassic - ~150 Ma.)

The first breakthrough was the 1860s discovery of Archaeopteryx, the first known feathered fossil. Thus, for over a century it has been a fundamental benchmark in paleontology - "the first bird." For roughly a century, no more fossils were recognized as being close to birds. But there were problems. We can call it a bird, but if we were to see a living one, it would seem immediately slightly non-birdy with its:

The second breakthrough came in 1964 - roughly a century after the discovery of Archaeopteryx when John Ostrom published his description of the dromaeosaurid ("raptor") Deinonychus which possessed many bird-like features (especially of the hand and wrist.)

In a totally separate issues, Ostrom raised the question of whether a creature that looked like Deinonychus could possibly be cold-blooded, questioning the prevailing dogma of dinosaur paleobiology. During the following decade, cladistic methods began to be applied to extinct vertebrates, supporting close relationships between birds and theropod (meat-eating) dinosaurs like Deinonychus.

The pigeon-sized deinonychosaurian Anchiornis huxleyi reconstructed with true colors
by Michael DiGiorgio from Grrlscientist
In the last 40 years, as more specimens of Archaeopteryx have been found and studied, and as our knowledge of small theropods has improved, the picture has gotten clearer:

The current consensus is that contour feathers and a range of other birdy characteristics characterize the theropod group Eumaniraptora. Basal members of the major eumaniraptoran groups were broadly similar:

But could any of them fly?

Their forelimb mechanics make this much clear: They could definitely flap. Biomechanically, Archaeopteryx is considered to be at or near the threshold of flight. Its wings are very short, its breast-bone is not large enough to serve as the origin for powerful flight muscles, and its skeleton was not strengthened and fused in the manner of modern birds. If it could fly, it did so weakly and over short distances. So why did it waste metabolic energy growing and supporting big feathered forelimbs? Indeed, what did Caudipteryx do with its dinky "wings?"

Turkey-sized oviraptorosaur Caudipteryx (left) and deinonychosaur Mei long
by Kahless28
Traditionally, paleontologists considered two hypotheses for the origin of bird flight: Wing assisted incline running (WAIR):
In 2003 research by Ken Dial of the University of Montana's Flight Laboratory revealed a locomotory behavior in modern birds not previously realized. Birds were discovered to run vertically up surfaces, aided by flapping their wings back and forth in order to generate traction against the surface. They called this behavior Wing Assisted Incline Running, or WAIR for short.

Note that these birds are NOT climbing in the typical sense: they are literally running up the sides of trees. Dial and his team studied the ontogenetic (growth) changes in the ability for birds to use this behavior, determining that WAIR is useful even in individuals with little dinky wings useless for flight. Sound familiar? We finally have a compelling evolutionary scenario for the evolution of wings from small theropod forelimbs.

Human-sized deinonychosaurian Deininychus
contemplates eating or befriending a wren by E Willoughby
In 2009, Denver Fowler of the Museum of the Rockies added a complimentary hypothesis, that flapping was used in flightless predatory eunamiraptorans like Velociraptor in order to stabilize the body on top of struggling prey, like modern birds of prey do. (Link to video cue to 2:30 minutes.)

Oddly, of the two major Eunamiraptoran groups, only Avialae stepped across the threshold of proper flight while deinonychosaurs backed away from it to become large flightless predators (right).

The take-home message:

Both bird and whale evolution document profound transitions in body form and ecology. For decades, these morphological gaps were matched by:

Together, these, provided ammunition for anti-evolutionists who claimed that they could never be bridged. Indeed, creationist tracts often referred directly to them as case studies in the absence of transitional forms. And yet, they have emerged as triumphs of evolutionary science thanks to synergies and reciprocal illumination between different analytic methods.

By its nature, the fossil record will always consist mostly of gaps. The wonder and glory is the real strength of the trend, over the last two-three decades, toward the narrowing of those gaps. In the case of whales, the emerging evidence is so good that it was cited by U.C. Berkeley paleontologist Kevin Padian as a case study of a compelling evolutionary story - a "poster-child" of evolution. Similar citations could be made to the developing stories of: