Written in Stone: Evolution, the Fossil Record, and Our Place in Nature

by Brian Switek

  Even scientists have sometimes searched for an evolutionary hero and constructed tales of how we came “up from the ape.” The honest desire to find such a character will ultimately be disappointed. There was never an “ascent of man,” no matter how desperately we might wish for there to be, just as there has not been a “descent of man” into degeneracy from a noble ancestor. We are merely a shivering twig that is the last vestige of a richer family tree. Foolishly, we have taken our isolation to mean that we are the true victors in life’s relentless race.

  Whether meaning is to be found in the heavens or in ourselves, we feel a pervasive need to ennoble our heritage. What else have we if we do not? History tells us that we are the descendants neither of an ape that intentionally strove to reach higher cerebral branches nor a holy couple created by divine fiat. Instead we are inheritors of a rare intelligence that can permeate the delicate workings of nature but fears what it might find there. There is no reason to fear. Life is most precious when its unity and rarity are recognized, and we are among the rarest of things.

  Time and Chance

  I returned, and saw under the sun, that the race is not to the swift, nor the battle to the strong, neither yet bread to the wise, nor yet riches to men of understanding, nor yet favour to men of skill; but time and chance happeneth to them all.

  —Ecclesiastes 9:11 (King James Version)

  Many years ago, when I was about five, my parents took me to see the dinosaurs at the American Museum of Natural History. This was before the great renovation of the paleontology exhibits in the 1990s. When I visited, the museum’s iconic Tyrannosaurus reared back as if doing a Godzilla impression, and the “Brontosaurus” loomed over the Jurassic Dinosaur Hall, complete with the wrong head. I was enthralled by the enormous skeletons that stood in the dim fossil halls, and as I stood in their long shadows my imagination ran away with me. I wondered what those old bones would have looked like when clothed in flesh, and in one fleeting moment, left alone in the quiet of the hall with my family, I could have sworn that I could just make out the dinosaurs’ distant rumbles and growls echoing across the depth of time.

  I still visit the museum’s dinosaurs every few months or so. Some of the childhood whimsy is gone, but they continue to fascinate me. Beyond the awesome grandeur of the reconstructed skeletons of Gorgosaurus , Triceratops, and other long-lost creatures, every single bone has a story to tell about the life and evolution of the animal it once belonged to. The skeletons of the dinosaurs are much more than static monuments of a bygone age; they are intricately detailed records of an ongoing evolutionary “experiment” that has been carried out on this planet for the past 3.5 billion years and provide the context for understanding our own evolutionary history.

  Perhaps, so long after the birth of paleontology, it is easy to forget what we have learned from such creatures, but by studying their remains we have begun to better understand the world we live in, from the fact of extinction to how life on earth came to be as it is today. Indeed, the history of the dinosaurs is particularly instructive because the mass extinction that wiped them out, among other creatures, sixty-five million years ago opened up new evolutionary opportunities for mammals, including our own early primate ancestors. But what if the asteroid that triggered the mass dying had missed the earth entirely? What would life look like today if, by some quirk of history, the dinosaurs had survived?

  An extended tenure for the dinosaurs might appear to be a subject better suited to science fiction than academic debate, but in 1982 paleontologist Dale Russell presented the “Dinosauroid,”—a scientifically derived vision of what a modern-day descendant of the maniraptoran dinosaur Troodon might look like.90,91 With its bipedal stance, grasping hands, forward-oriented eyes, and relatively large brain, Troodon looked to Russell to be evolving towards a humanlike condition. Since Russell believed that our species possesses the only body capable of housing our complex minds, he affirmed that the lineage of Troodon—had it continued to evolve high levels of intelligence—would have been molded into something like us. With the help of artist Ron Seguin, Russell created a full-sized sculpture of his thought experiment, the end result being a creature that looked a more refined version of the sleestaks that menaced the heroes of the 1970s television series Land of the Lost.

  Though it would be a stretch to call the Dinosauroid beautiful, it did stroke our vanity. It was an assurance that evolution guaranteed the arrival of something like us, if not exactly our species, by way of natural law. Never mind that we now know that close relatives of Troodon survived the mass extinction in the form of birds (some of which, such as crows, are highly intelligent animals with an anatomy very different from our own). The Dinosauroid was an attempt not so much to ask what life might be like now had the extinction been canceled but to affirm that there is a sort of “human niche,” which would inevitably be filled by one kind of creature or another in time. We could feel confident that our species had practically been foreordained from the time the first life emerged. Given how easily this can be squared with the beliefs of human origins as told by Abrahamic religions it is not surprising that a number of scientists who hold theistic beliefs (such as Russell, Francis Collins, Kenneth Miller, and Simon Conway Morris) have advocated similar versions of human inevitability. Rather than being open-ended, evolution would be a process so constrained by the laws of physics and chemistry that, if we had the ability to restart evolution from the beginning, the history of life would end up looking much the same. In this light it could be believed that the emergence of Homo sapiens, the single remaining species of hominin on a backwater planet far removed from the center of the galaxy or even the universe, was woven into the very fabric of the universe itself.

  But our human conceit blinds us to the true pattern of the fossil record. The diversity of life and the disparity between organisms cannot solely be explained by a lawlike evolutionary mechanism driving organisms up a ladder of predictable progress. Instead, the fossil record has revealed the growth of the tree of life to be much more haphazard; a directionless unfolding greatly influenced by phenomena from catastrophic mass extinctions to the emergence of new forms thanks to the co-option of the old structures into new ones. Tetrapods carry that name because of an ancient duplication error which caused some fish to have four fins instead of two; the inner ear bones of mammals evolved because of the close connection between ear and jaw in their ancestors; whales swim in their peculiar fashion because of the way their ancestors moved on land; and many of the features which allowed birds to take to the air evolved first in ground-dwelling dinosaurs. These changes were not effected in a linear fashion, as if creatures were striving for a particular goal. Instead, as shown by the evolutionary radiations of fossil elephants, horses, and humans, species branch out over time, oftentimes with descendants living alongside their ancestors. As time goes on, some of these branches are pruned back by extinction while others persist. In our case, especially, the fossil record has unequivocally shown that what was once a diverse family of humans has been reduced to just one species, leaving us good reason to wonder what might happen to our kind in the future.

  This view of life, in which we owe our existence to a long series of antecedent states molded by unpredictable events, has not often been welcomed. It cuts to the core of what we have traditionally held up as human exceptionalism by inextricably tying us to quirks of history which could have gone another way. Had the first land-dwelling vertebrates six legs instead of four, had the Permian extinction extirpated the cynodonts, had the asteroid which struck earth sixty-five million years ago missed, or had our ape ancestors clung to an arboreal lifestyle in equatorial forests—there can be no doubt that life on earth today would be very different. If we could somehow rewind and replay earth’s history, as suggested by Stephen Jay Gould in his book Wonderful Life, we would not expect life to follow the same history with which we are now familiar. Entirely different organisms would probably evolve and perish over the course of history, wit
h the appearance of anything like our species evolving extremely slim. Unfortunately we cannot actually run Gould’s thought experiment, nor have we yet discovered life elsewhere in the universe with which to compare organisms from our own planet, but an ingenious laboratory experiment has suggested that history and happenstance have key roles to play in evolution.

  In 1988, the biologist Richard Lenski founded twelve populations of identical Escherichia coli bacteria in his lab at the University of Michigan. To keep track of how the populations were changing, the scientists in the lab froze representative samples of the populations every 500 generations (or about every two and a half months). This provided them with a preserved evolutionary record of each group that could be revitalized to see if evolution would repeat itself from a given starting point. By 2008, over 44,000 generations of the bacteria had lived and died in the lab, and it was in that year that Lenski (along with co-authors Zachary Blount and Christina Borland) reported on a major change that had recently occurred in the populations.

  From the beginning of the experiment, the E. coli populations were kept in environments with only the minimum materials needed to keep them alive, which also included a resource called citrate. Although citrate can be an important component of the citric acid cycle in cells, E. coli cannot transport it across their cell membranes when oxygen is present, so while each of the twelve populations was kept with a store of this useful resource none of them could actually make use of it. As had been expected, the starting populations of E. coli were not able to utilize the citrate in their environment, and even though the bacteria were accumulating mutations none gave any of the populations the variability needed to use citrate. By the 31,500 generation mark, however, one population had evolved the ability to use the citrate—but what had caused this change? Did this happen because of a single-point mutation that immediately opened up new possibilities, or did the mutation build upon previous changes to allow the bacteria a new ability?

  To find out, the researchers went back to their library of bacteria. Using clones from twelve different points in the study, for a total of 72 “replay” populations in all, the scientists let the bacteria reproduce for about 3,700 generations, checking along the way to see if citrate-using populations evolved again. What they found was that the origin of citrate-using bacteria was contingent upon a series of chance events. The first was a mutation that modified the “genetic background” of the bacteria so that a later mutation would be more likely to produce a citrate-using form. It had gone undetected as it did not have an immediately visible effect on the bacteria. A second mutation, around generation 30,000, allowed some bacteria to start using citrate, and soon afterwards another mutation made the citrate-using bacteria more efficient at using the resource (while simultaneously reducing their ability to utilize glucose as their ancestors in the first generation did).

  Even though the ability to use citrate would have been advantageous for any population of bacteria, it took well over a decade, more than 30,000 generations, for that adaptation to appear, and thus far it has only appeared in a single population out of twelve. It was not caused by an extremely rare single event but was precipitated from a succession of prior conditions that allowed the change to take place. Since these changes did not appear in the other eleven populations of bacteria, all grown under identical conditions from the same starting point, the discovery suggests that evolution does not run according to the same path every time. The element of chance, as embodied by mutations and other genetic changes, can open or close potentials for organisms that are then molded by natural selection. As the authors of the study concluded:Our study shows that historical contingency can have a profound and lasting impact under the simplest, and thus most stringent, conditions in which initially identical populations evolve in identical environments. Even from so simple a beginning, small happenstances of history may lead populations along different evolutionary paths. A potentiated cell took the one less traveled by, and that has made all the difference.

  Unfortunately, we do not have a frozen store of early hominins, Permian synapsids, Devonian tetrapods, or other prehistoric vertebrates ready to be thawed out to run similar experiments, but what was found in the Lenski lab certainly applies to the fossil record. As the biologist Jacques Monod is reported to have said, “What is true for E. coli is also true for the elephant.” (And also for us!) Small, unpredictable changes can have major influences on how life evolves, cordoning off some evolutionary potentials and allowing for others. Organisms are constrained not only by the chemistry and physics on this planet, from gravity to the composition of the atmosphere, but by the role of chance in their own history.

  The complementary roles of contingency and constraint in life’s history are among the central lessons of the fossil record. Looking back from our narrowed present perspective we can make out a branching pattern, marked by fits and starts, in which each successively older slice of life’s history only makes sense in the context of what came before it. It is comprehensible, but not arranged according to a predetermined plan, and there is no sign that evolution is being pushed along blessed roads of increasingly perfect adaptations put in place from the very beginning. Life as it is now did not have to be.

  We are constantly grappling with our place in nature. In the over 4.6-billion-year history of our planet and the over three billion years that life has existed on earth, organisms like us and our hominin cousins have only evolved once, and at a relatively late date, at that. Rather than suggest that creatures like us were somehow predestined to be, this fact of the record means that, much like the bacteria in Lenski’s lab, hominins are the rare products of a contingent process that probably would not evolve again if evolution were restarted. Some might find this hard to accept. Life-forms in innumerable splendid varieties have flourished and been extinguished on this planet, and yet we still feel compelled to find some reason, any reason, that our existence was premeditated and imbued with special meaning. No one argues the case for the inevitability of maple trees or damselflies or aardvarks; it is always our species that is cast as the specially intended pinnacle and purpose of nature. Such hubris is absurd. As Mark Twain jested in his essay “Was the World Made for Man?”:Man has been here 32,000 years. That it took a hundred million years to prepare the world for him is proof that that is what it was done for. I suppose it is. I dunno. If the Eiffel Tower were now representing the world’s age, the skin of paint on the pinnacle-knob at its summit would represent man’s share of that age; and anybody would perceive that that skin was what the tower was built for. I reckon they would, I dunno.

  If we can let go of our conceit, we may find that an understanding of evolution makes life on this planet all the more precious. Every living species is a unique, still-changing part of lineages that have persisted for billions of years, and once they are gone they are lost forever. The same is true of our species. Sooner or later, extinction claims all.

  Nothing quite like us has ever existed on earth before and may not ever again after we are gone. Given the contingencies of our own history, that we exist at all is amazing. If we wish to know ourselves, we must understand our history. We are creatures of time and chance.

  Notes

  1 Thankfully the destruction of the Messel pit was stopped. The quarry was given the status of a UNESCO World Heritage site in 1995. Private collectors are no longer allowed to remove fossils, but scientific research continues.

  2 A few journalists, such as Science anthropology writer Ann Gibbons, were able to wrangle a look at early versions of the paper, but they had to sign non-disclosure agreements before doing so.

  3 This conclusion was supported by a second paper—submitted for publication before publication of the description of Afradapis—which appeared in March 2010 in the Journal of Human Evolution. Written by early primate experts Blythe Williams, Richard Kay, Christopher Kirk, and Callum Ross, the paper reaffirmed that Darwinius had far more in common with lemurs than monkeys.

  4 My
miniscule contributions to the public discussion about Darwinius included several blog posts, two pieces in the Times of London, and two appearances on the BBC Radio 4 show Material World. I hope they helped at least a few people understand that Ida was not all she was cracked up to be.

  5 Though, as Adams so astutely pointed out, perhaps we should be more concerned with asking the right questions first.

  6 If something that was imaginable didn’t exist, it was argued, the thing either had not been found or possessed some inherent contradiction that barred the actual possibility of its existence.

  7 There are some notable exceptions, of course, such as the Blob (a living, oozing metaphor for communism) and the parasitic stars of the Alien series, but for every such creature there seems to be a score of big-brained humanoids.

  8 In the interest of fairness, however, it should be noted that Dawkins does consider the fossil record as evidence for evolution in his latest book, The Greatest Show on Earth.