Brilliant Blunders: From Darwin to Einstein - Colossal Mistakes by Great Scientists That Changed Our Understanding of Life and the Universe (4 page)

Darwin borrowed the idea embodied in his second pillar, that of
gradualism, mainly from the works of two geologists. One was the eighteenth-century geologist James Hutton, and the other was Darwin’s contemporary and later close friend Charles Lyell. The geological record showed horizontal banding patterns covering large geographical areas. This, coupled with the uncovering of different fossils within these bands, suggested a progression of incremental change. Hutton and Lyle were largely responsible for the formulation of the
modern theory of
uniformitarianism:
the notion that the rates at which processes such as erosion and sedimentation occur at present are similar to the rates in the past. (We shall return to this concept in chapter 4, when we’ll discuss Lord Kelvin.) Darwin argued that just as geological action shapes the Earth gradually but surely, evolutionary changes are the result of transformations that span hundreds of thousands of generations. One should not, therefore, expect to see significant alterations in less than tens of thousands of years, except perhaps in organisms that multiply very frequently, such as bacteria, which, as we know today, can develop resistance to antibiotics in extremely short times. Contrary to uniformitarianism, however, the rate of evolutionary changes is generally nonuniform in time for a given species, and it can vary further from one species to another. As we shall see later, it is the pressure exerted by natural selection that determines primarily how fast evolution manifests itself.
Some “living fossils” such as the lamprey—a jawless marine vertebrate with a funnel-like mouth—appear to have hardly evolved in 360 million years. As a fascinating aside, I should note that the idea of gradual change was put forth in the seventeenth century by the empiricist philosopher John Locke, who wrote insightfully, “The boundaries of the species, whereby men sort them, are made by men.”

The next pillar in Darwin’s theory,
the concept of a
common ancestor
, is what has become in its modern incarnation the primary motivator for all of the present-day searches for the origin of life. Darwin first argued that there is no doubt that all the members of any taxonomic class—such as all vertebrates—originated from a common ancestor. But his imagination carried him much further with this concept. Even though his theory predated any knowledge
of the facts that all living organisms share such characteristics as the DNA molecule, a small number of amino acids, and the molecule that serves as the currency for energy production, Darwin was still bold enough to proclaim, “Analogy would lead me one step further, namely, to the belief that all animals and plants have descended from some one prototype.” Then, after cautiously acknowledging that “analogy may be a deceitful guide,” he still concluded that “probably all the organic beings which have ever lived on the earth have descended from some one primordial form, into which life was first breathed.”

But, you may wonder, if all life on Earth originated from a single, common ancestor, how did the astonishing wealth of diversity arise? After all, this was the first hallmark of life that we have identified as one that requires an explanation. Darwin did not flinch, and took this challenge head-on—it was not an accident that the title of his book had the word “species” in it.
Darwin’s solution to the diversity problem involved another original idea: that of branching, or speciation. Life starts from a common ancestor, just as a tree has a single trunk, Darwin reasoned. In the same way that the trunk develops branches, which then split into twigs,
the “tree of life” evolves by many branching and ramification events, creating separate species at each splitting node. Many of these species become extinct, just like the dead and broken branches of a tree. However, since at each splitting the number of offspring species from a given ancestor doubles, the number of different species can increase dramatically. When does speciation actually occur? According to modern thinking, mainly when a group of members of a particular species becomes geographically separated. For instance, one group may wander to the rainy side of a mountain range, while the rest of the species stays on the dry slope. Over time, these rather different environments produce different evolutionary paths, eventually leading to two populations that can no longer interbreed—or in other words, different species. In rarer occasions, speciation could create new species that arise from interbreeding between two species. Such appears to have been
the case of the Italian sparrow, which was shown in 2011 to be
genetically intermediate between Spanish sparrows and house sparrows. Italian and Spanish sparrows behave like distinct species, but Italian and house sparrows do form hybrid zones, where the ranges of the two interbreeding species meet.

Amazingly, in 1945,
author Vladimir Nabokov, of
Lolita
and
Pale Fire
fame, came up with a sweeping hypothesis for the evolution of a group of butterflies known as the Polyommatus blues. Nabokov, who had a lifelong interest in butterflies, speculated that the butterflies came to the New World from Asia in a series of waves lasting millions of years. To their surprise, a team of scientists using gene-sequencing technology confirmed Nabokov’s conjecture in 2011. They found that the New World species shared a common ancestor that lived about ten million years ago, but that many New World species were more closely related to Old World butterflies than to their neighbors.

Darwin was sufficiently aware of the importance of the concept of speciation to his theory to include a schematic diagram of his tree of life. (Figure 3 shows the original drawing from his 1837 notebook.) In fact, this is the only figure in the entire book. Fascinatingly, Darwin included the caveat “I think” at the top of the page!

In many cases, evolutionary biologists have been able to identify most of the intermediate steps involved in speciation: from pairs of species that have probably recently split from a single species, to pairs that are just about ready to be pushed into separation. At the more detailed level, a combination of molecular and fossil data has yielded, for instance, a relatively well-resolved and well-dated
phylogenetic tree for all the families of living and very recently extinct mammals.

I cannot refrain at this point from digressing to note that from my own personal perspective, there is another aspect of the notions of a common ancestor and of speciation that makes Darwin’s theory truly special. About a decade ago, while working on the book
The Accelerating Universe,
I was trying to identify the ingredients that make a physical theory of the universe “beautiful” in the eyes of scientists. In the end, I concluded that two of the absolutely
essential constituents were
simplicity
and something that is known as the
Copernican principle.
(In the case of physics, the third ingredient was
symmetry.
) By “simplicity,”
I mean reductionism, in the sense that most physicists understand it: the ability to explain as many phenomena as possible with as few laws as possible. This has always been, and still is, the goal of modern physics. Physicists are not satisfied, for instance, with having one extremely successful theory (quantum mechanics) for the subatomic world, and one equally successful theory (general relativity) for the universe at large. They would like to have one unified “theory of everything” that would explain it all.

Figure 3

The Copernican principle derives its name from that of the Polish astronomer Nicolaus Copernicus, who in the sixteenth
century removed the Earth from its privileged position at the center of the universe. Theories that obey the Copernican principle do not require humans to occupy any special place for these theories to work. Copernicus taught us that the Earth is not at the center of the solar system, and all the subsequent findings in astronomy have only strengthened our realization that, from a physics perspective, humans play no special role in the cosmos. We live on a tiny planet that revolves around an ordinary star, in a galaxy that contains hundreds of billions of similar stars. Our physical insignificance continues even further. Not only are there about two hundred billion galaxies in our observable universe, but even ordinary matter—the stuff that we and all the stars and gas in all the galaxies are made of—constitutes only a little over 4 percent of the universe’s energy budget. In other words, we are really nothing special. (In chapter 11 I will discuss some ideas suggesting that we should not take Copernican modesty too far.)

Both reductionism and the Copernican principle are the true trademarks of Darwin’s theory of evolution. Darwin explained just about everything related to life on Earth (except its origin) with one unified vision. One can hardly be more reductionistic than that. At the same time, his theory was Copernican to the core. Humans evolved just like every other organism. In the tree analogy, all of the youngest buds are separated from the main trunk by a similar number of branching nodes, the only difference being that they point in different directions. Equivalently, in Darwin’s evolutionary scheme, all the present-day living organisms, including humans, are the products of similar paths of evolution. Humans definitely do not occupy any exceptional or unique place in this scheme—they are not the lords of creation—but an adaptation and development of their ancestors on Earth. This was the end of “absolute anthropocentrism.” All the terrestrial creatures are part of the same big family. In the words of the influential evolutionary biologist Stephen Jay Gould, “Darwinian evolution is a bush, not a ladder.” To a large extent, what has fueled the opposition to Darwin for more than 150 years is precisely this fear that the theory of evolution
displaces humans from the pedestal on which they have put themselves. Darwin has initiated a rethinking of the nature of the world and of humans. Note that in a picture in which only the “fittest” survive (as we shall soon discuss in the context of natural selection), one could argue that insects have clearly outclassed humans, since there are so many more of them. Indeed,
the British geneticist J. B. S. Haldane is cited (possibly apocryphally) as having replied to theologians who inquired whether there was anything that could be concluded about the Creator from the study of creation, with the observation that God “has an inordinate fondness for beetles.” Today we know that even in terms of genome size—the entirety of the hereditary information—humans fall far short of, believe it or not,
a fresh water ameboid named
Polychaos
dubium
. With 670 billion base pairs of DNA reported, the genome of this microorganism may be more than two hundred times larger than the human genome!

Darwin’s theory, therefore, amply satisfies the two applicable criteria (which admittedly are somewhat subjective) for a truly beautiful theory. No wonder, then, that
The Origin
has elicited perhaps the most dramatic shift of thought ever brought about by a scientific treatise.

Returning now to the theory itself, Darwin was not content with merely making statements about evolutionary changes and the production of diversity. He regarded it as his main task to explain
how
these processes have occurred. To achieve this goal, he had to come up with a convincing alternative to creationism for the apparent design in nature. His idea—natural selection—has been esteemed by Tufts University philosopher Daniel C. Dennett as no less than “the single best idea anyone has ever had.”

One of the challenges that the concept of evolution posed concerned adaptation: the observation that species appeared to be perfectly harmonized with their environments, and the mutual adaptedness of the traits of organisms—body parts and physiological processes—to
one another. This created a puzzle that confounded even the evolutionary minded among the naturalists that preceded Darwin: If species are so well adapted, how could they evolve and still remain well adapted? Darwin was fully aware of this conundrum, and he made sure that his principle of natural selection provided a satisfactory solution.

The basic idea underlying natural selection is quite simple (once it is pointed out!). As it sometimes happens with discoveries whose time has come, the naturalist Alfred Russel Wallace independently formulated very similar ideas at about the same time. Wallace was nevertheless very clear on who he thought deserved most of the credit.
In a letter to Darwin on May 29, 1864, he wrote:

As to the theory of Natural Selection itself, I shall always maintain it to be actually yours and yours only. You had worked it out in details I had never thought of, years before I had a ray of light on the subject, and my paper would never have convinced anybody or been noticed as more than an ingenious speculation, whereas your book has revolutionized the study of Natural History.