Evolution – what scientists say

Could it have happened?

All you need is faith.

Given its importance and pervasiveness, the processes underlying evolutionary innovation are, however, remarkably poorly understood, which leaves us at a surprising conundrum: while biologists have made great progress over the past century and a half in understanding how existing traits diversify, we have made relatively little progress in understanding how novel traits come into being in the first place.

Armin Moczek* et al., 2008. BioEssays 30, 432–447.
*Department of Biology, Indiana University

The Origin of Life. This problem is one of the big ones in science. It begins to place life, and us, in the universe. Most chemists believe, as do I, that life emerged spontaneously from mixtures of molecules in the prebiotic Earth. How? I have no idea. … On the basis of all the chemistry that I know, it seems to me astonishingly improbable.

George M. Whitesides*, 2007. Chemical and Engineering News 85, 12–17.
*Department of Chemistry and Chemical Biology, Harvard University

Nature is even more wondrous than the architects of the Modern Synthesis thought, and involves processes we thought were impossible.
Denis Noble* et al., 2014. Journal of Physiology 592, 2237–2244.
*University of Oxford

Where and how did the complex genetic instruction set programmed into DNA come into existence? The genetic set may have arisen elsewhere and was transported to the Earth. If not, it arose on the Earth, and became the genetic code in a previous lifeless, physical–chemical world. Even if RNA or DNA were inserted into a lifeless world, they would not contain any genetic instructions unless each nucleotide selection in the sequence was programmed for function. Even then, a predetermined communication system would have had to be in place for any message to be understood at the destination. Transcription and translation would not necessarily have been needed in an RNA world. Ribozymes could have accomplished some of the simpler functions of current protein enzymes. Templating of single RNA strands followed by retemplating back to a sense strand could have occurred. But this process does not explain the derivation of “sense” in any strand. “Sense” means algorithmic function achieved through sequences of certain decision-node switch-settings. These particular primary structures determine secondary and tertiary structures. Each sequence determines minimum-free-energy folding propensities, binding site specificity, and function. Minimal metabolism would be needed for cells to be capable of growth and division. All known metabolism is cybernetic – that is, it is programmatically and algorithmically organized and controlled.

J.T. Trevors* & D.L. Abel**, 2004. Cell Biology International 28, 729–739
*Department of Environmental Biology, University of Guelph, Ontario
**The Gene Emergence Project, The Origin-of-Life Foundation Inc., Maryland

One of biology’s most significant unresolved issues is to understand how novel, complex phenotypes originate, both developmentally and evolutionarily.

Cris Ledon-Rettig* et al., 2008. Evolution & Development 10, 316–325.
*Department of Biology, University of North Carolina

The skeletal architecture of vertebrates is widely divergent, yet the basis for change in gross skeletal morphology remains almost entirely unknown.

David Rudel* & Ralf Sommer*, 2003. Developmental Biology 264, 15–37.
*Max-Planck Institut für Entwicklungsbiologie, Tübingen

There is still much to do in order to fully understand how novel complex traits evolve. … This work is difficult and time consuming, but the question at its core—the genetic origin of new and complex traits—is probably still one of the most pertinent and fundamental unanswered questions in evolution today.

Antónia Monteiro* & Ondrej Podlaha*, 2009. PLoS Biology 7, e1000037.
*Department of Ecology and Evolutionary Biology, Yale University

Although animals display a rich variety of shapes and patterns, the genetic changes that explain how complex forms arise are still unclear.

Arnaud Martin* et al., 2012. Proceedings of the National Academy of Sciences USA 109, 12632–12637.
*Department of Ecology and Evolutionary Biology, University of California

How body pattern evolves in nature remains largely unknown.

Philip Cleves* et al., 2014. Proceedings of the National Academy of Sciences USA 111, 13912–13917.
*Department of Molecular and Cell Biology, University of California

Picking the right tree is challenging. “For 15 years, people hoped for some software that could solve these problems at the push of a button, but sadly that’s not come to pass. It’s a very, very difficult problem algorithmically,” says Boore. It is especially hard to say whether traits were inherited from a common ancestor, whether they look similar but evolved independently in unrelated groups – such as wings of birds and bats – or whether they might have evolved but were then lost in later descendants, such as eyes in cave-dwelling fish. Often several different trees are equally good explanations of the data. And different genes from the same set of organisms often predict different trees.

John Whitefield, 2007. Nature 446, 247–249.

The explanation for adaptation is natural selection. We are not yet sure what the explanation for novelties is. … I suspect that the origin of novelties also requires natural selection as well as additional mechanisms, but what they are will have to be determined by more empirical research.

Günter Wagner, 2014. Homology, Genes, and Evolutionary Innovation.
*Department of Ecology and Evolutionary Biology, Yale University

Our theory of evolution has become … one which cannot be refuted by any possible observation. Every conceivable observation can be fitted into it. It is thus ‘outside of empirical science’ but not necessarily false. No one can think of ways in which to test it. Ideas, either without basis or based on a few laboratory experiments carried out in extremely simplified training have attained currency far beyond their validity. They have become part of an evolutionary dogma accepted by most of us as part of our training.

L.C. Birch* and P.R. Ehrlich**, 1967. Nature 214, 349–352
*School of Biological Sciences, University of Sydney
Department of Biological Sciences, Stanford University

Did it happen?

Most families, orders, classes, and phyla appear rather suddenly in the fossil record, often without anatomically intermediate forms smoothly interlinking evolutionarily derived descendant taxa with their presumed ancestors.

Niles Eldredge*, 1989. Macro-Evolutionary Dynamics: Species, Niches, and Adaptive Peak
*American Museum of Natural History, New York

The fossil record suggests that the major pulse of diversification of phyla occurs before that of classes, classes before that of orders, and orders before families. This is not to say that each higher taxon originated before species (each phylum, class, or order contained at least one species, genus, family, etc. upon appearance), but the higher taxa do not seem to have diverged through an accumulation of lower taxa. … We usually do not see representative intermediates, even within groups that are durably skeletonized, before and after the origin of higher taxa.

Douglas Erwin* et al., 1987. Evolution 41, 1177–1186.
*Department of Geological Sciences, Michigan State University

It is as though they were just planted there, without any evolutionary history. Needless to say this appearance of sudden planting has delighted creationists.

Richard Dawkins*, 1987. The Blind Watchmaker.
*Department of Zoology, University of Oxford

Described recently as ‘the most important evolutionary event during the entire history of the Metazoa,’ the Cambrian explosion established virtually all the major animal body forms — Baupläne or phyla — that would exist thereafter, including many that were ‘weeded out’ and became extinct. Compared with the 30 or so extant phyla, some people estimate that the Cambrian explosion may have generated as many as 100. The evolutionary innovation of the Precambrian/Cambrian boundary had clearly been extremely broad: ‘unprecedented and unsurpassed,’ as James Valentine of the University of California, Santa Barbara, recently put it. … Why, in subsequent periods of great evolutionary activity when countless species, genera, and families arose, have there been no new animal body plans produced, no new phyla?

Roger Lewin, 1988. Science 241, 291

If any event in life’s history resembles man’s creation myths, it is this sudden diversification of marine life when multicellular organisms took over as the dominant actors in ecology and evolution. Baffling (and embarrassing) to Darwin, this event still dazzles us and stands as a major biological revolution on a par with the invention of self-replication and the origin of the eukariotic cell. The animal phyla emerged out of the Precambrian mists with most of the attributes of their modern descendants.

Stefan Bengston*, 1990. Nature 345, 765–766.
*Institute of Palaeontology, Uppsala, Sweden

Taxa recognized as orders during the (Precambrian-Cambrian) transition chiefly appear without connection to an ancestral clade via a fossil intermediate. This situation is in fact true of most invertebrate orders during the remaining Phanerozoic as well. There are no chains of taxa leading gradually from an ancestral condition to the new ordinal body type. Orders thus appear as rather distinctive subdivisions of classes rather than as being segments in some sort of morphological continuum.

James W. Valentine* et al. 1991. Evolutionary Biology 25, 279–356
*Department of Integrative Biology, University of California

All of the basic architectures of animals were apparently established by the close of the Cambrian explosion; subsequent evolutionary changes, even those that allowed animals to move out of the sea onto land, involved only modifications of those basic body plans.

Douglas H. Erwin*. American Scientist, March/April 1997.
*Smithsonian Institution National Museum of Natural History

The only illustration Darwin published in On the Origin of Species was a diagram depicting his view of evolution: species descendant from a common ancestor; gradual change of organisms over time; episodes of diversification and extinction of species. Given the simplicity of Darwin’s theory of evolution, it was reasonable for paleontologists to believe that they should be able to demonstrate with the hard evidence provided by fossils both the thread of life and the gradual transformation of one species into another. Although paleontologists have, and continue to claim to have, discovered sequences of fossils that do indeed present a picture of gradual change over time, the truth of the matter is that we are still in the dark about the origin of most major groups of organisms. They appear in the fossil record as Athena did from the head of Zeus – full-blown and raring to go.

Jeffrey H. Schwartz*, 1999. Sudden Origins: Fossils, Genes, and the Emergence of Species
*Departments of Anthropology and History and Philosophy of Science, University of Pittsburgh

The most striking features of large-scale evolution are the extremely rapid divergence of lineages near the time of their origin, followed by long periods in which basic body plans and ways of life are retained. What is missing are the many intermediate forms hypothesized by Darwin, and the continual divergence of major lineages into the morphospace between distinct adaptive types. The most conspicuous event in metazoan evolution was the dramatic origin of major new structures and body plans documented by the Cambrian explosion. Until 530 million years ago, multicellular animals consisted primarily of simple, soft-bodied forms, most of which have been identified from the fossil record as cnidarians and sponges. Then, within less then 10 million years, almost all of the advanced phyla appeared, including echinoderms, chordates, annelids, brachiopods, molluscs and a host of arthropods.

Robert L. Carroll*, 2000. Trends in Ecology & Evolution 15, 27-32
*Department of Biology, McGill University, Montreal

As Darwin noted in the Origin of Species, the abrupt emergence of arthropods in the fossil record during the Cambrian presents a problem for evolutionary biology. There are no obvious simpler or intermediate forms—either living or in the fossil record—that show convincingly how modern arthropods evolved from worm-like ancestors. Consequently there has been a wealth of speculation and contention about relationships between the arthropod lineages.

Daniel Osorio*, Jonathan P. Bacon, Paul M. Whitington, 1997. American Scientist 85/3, 244-253
*School of Biological Sciences, Sussex University, UK

Fuxianhuia protensa … suggests that the organization we see in the modern [arthropod] brains is very ancient,” says coauthor Nicholas Strausfeld, a neuroscientist at the University of Arizona. Scientists had thought that early arthropods had simpler brains like those of tiny freshwater crustaceans called branchiopods. The branchiopod brain consists of two connected parts with a third mass of tissue sitting behind the stomach. Sometime after the branchiopod lineage split from other arthropods, scientists had assumed, the nervous tissue behind the gut migrated and connected with the other parts of the brain. “With this complex Cambrian brain, we have to rethink our current inter pretation,” says Steffen Harzsch of the University of Greifswald in Germany. The ancestors of branchiopods probably had a more complicated brain originally and later did some evolutionary back pedaling, he says.

Erin Wayman, reporting a paper in Nature for Science News, 17 Nov 2012

Analyses of the fossil orders show a burst of [insect] origination in the Late Carboniferous … possibly associated with the appearance of the first winged insects in the fossil record.

Fabien Condamine* et al. 2016. Scientific Reports 6, 19208.
*French National Centre for Scientific Research, Villejuif, France

We have a great deal of knowledge of the anatomy of a vast array of Paleozoic tetrapods … but the specific interrelationships of the major taxa and their affinities with the modern orders remain impossible to establish with assurance.

Robert L. Carroll* 2001. J. Paleontology 75, 1202–1213.
*McGill University, Montreal

Our picture of marine diversity through geologic time is based mainly on the relatively continuous fossil record of animals with shells. However, some 19 (~60%) of the ~33 phyla recognized today lack readily preserved hard parts and are soft-bodied. Exceptionally preserved Cambrian fossil deposits, such as the famous Burgess Shale, yield examples of 14 of the 19 soft-bodied phyla. If today’s representatives provide a reliable guide, the small size and fragile nature of members of the other five mitigated against preservation, except in unlikely contexts, and explain why they appear much later in the fossil record. … We have an unusually complete window on the marine forms that evolved at this critical juncture in the history of animal life.

Derek E.G. Briggs, 2015. Current Biology 25, R845–R875
*Department of Geology and Geophysics Yale University

From Charles Darwin onward, evolutionary biologists have been perplexed by the apparently instantaneous first appearances of numerous phyla (a highly disparate sample of species) in the Cambrian fossil record. The subsequent discovery of hitherto unknown fossil groups from the Cambrian Burgess Shale and similar localities added to the enigma, prompting the radical hypothesis that the disparity of metazoans peaked in the Cambrian and subsequent extinctions winnowed this down to much more modest levels soon thereafter. … Cambrian animal groups had a disparity comparable to that of their modern counterparts. [The] Precambrian fossil record is enigmatic at best.

Martin Hughes* et al. 2013. Proc. Nat. Acad. Sci. 110, 13875–13879.
*University of Bath, UK

The dominance of dinosaurs ensured that our distant mammalian ancestors remained no larger than a cat. But when a catastrophic asteroid or comet—maybe a few comets, as some scientists are now arguing—finished off the dinosaurs 65 million years ago, mammals got the most important evolutionary opportunity they would ever have. … Within a few million years of the impact the fossil record shows an explosion in mammalian diversity. … How did those little creatures transform into not only the hippo and the mole rat but also today’s vast panorama of mammals with fur, hooves, and fangs, as well as others that swim hairless through deep oceans?… That question has never had an easy answer. … Fossils suggest that most modern groups appeared around 60 million years ago, after the dinosaurs were gone. Molecular data suggest they actually began diversifying about 100 million years ago. … Many paleontologists angrily reject the DNA findings, arguing there must be something wrong with the molecular clocks the geneticists use to date their findings. The geneticists counter that paleontologists just haven’t found the right fossils yet.

Rick Gore, April 2003. National Geographic

It is not until about 55 million years ago that we begin to clearly see placental mammal lineages. Why did virtually all placental groups—such as primates, bats, ungulates, and whales—appear so abruptly in the fossil record? Where are the transitional forms that must link the diminutive insectivores of the Mesozoic to today’s multitude of mammals?

Anne Yoder* 2013, Science 339, 656–658.
*Duke University, Durham, NC

In the opaque language of Erwin et al. (op. cit. 1987):

Families were exploiting open opportunities within adaptive spaces to which higher-level taxa had [already] achieved access.

Animals were hitting the fossil record as they multiplied and radiated across an empty world.
In the simple language of the Genesis tradition:

In the second month, on the twenty-seventh day of the month, the earth had dried out. Then God said to Noah, “Go out from the ark, you and your wife, and your sons and your sons’ wives with you. Bring out with you every living thing that is with you of all flesh—birds and animals and every creeping thing that creeps on the earth—that they may swarm on the earth, and be fruitful and multiply on the earth.” So Noah went out, and his sons and his wife and his sons’ wives with him. Every beast, every creeping thing, and every bird, everything that moves on the earth, went out by families from the ark.