Fossil Record Evolution Essay

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As a biological anthropologist, I never liked drawing sharp distinctions between human and non-human. Such boundaries make little evolutionary sense, as they ignore or grossly underestimate what we humans have in common with our ancestors and other primates. What's more, it's impossible to make sharp distinctions between human and non-human in the paleoanthropological record. Even with a time machine, we couldn't go back to identify one generation of humans and say that the previous generation contained none: one's biological parents, by definition, must be in the same species as their offspring. This notion of continuity is inherent to most evolutionary perspectives and it's reflected in the similarities (homologies) shared among very different species. As a result, I've always been more interested in what makes us similar to, not different from, non-humans.

Evolutionary research has clearly revealed that we share great biological continuity with others in the animal kingdom. Yet humans are truly unique in ways that have not only shaped our own evolution, but have altered the entire planet. Despite great continuity and similarity with our fellow primates, our biocultural evolution has produced significant, profound discontinuities in how we interact with each other and in our environment, where no precedent exists in other animals. Although we share similar underlying evolved traits with other species, we also display uses of those traits that are so novel and extraordinary that they often make us forget about our commonalities. Preparing a twig to fish for termites may seem comparable to preparing a stone to produce a sharp flake—but landing on the moon and being able to return to tell the story is truly out of this non-human world.

Humans are the sole hominin species in existence today. Thus, it's easier than it would have been in the ancient past to distinguish ourselves from our closest living relatives in the animal kingdom. Primatologists such as Jane Goodall and Frans de Waal, however, continue to clarify why the lines dividing human from non-human aren't as distinct as we might think. Goodall's classic observations of chimpanzee behaviors like tool use, warfare and even cannibalism demolished once-cherished views of what separates us from other primates. de Waal has done exceptional work illustrating some continuity in reciprocity and fairness, and in empathy and compassion, with other species. With evolution, it seems, we are always standing on the shoulders of others, our common ancestors.

Primatology—the study of living primates—is only one of several approaches that biological anthropologists use to understand what makes us human. Two others, paleoanthropology (which studies human origins through the fossil record) and molecular anthropology (which studies human origins through genetic analysis), also yield some surprising insights about our hominin relatives. For example, Zeresenay Alemsegad's painstaking field work and analysis of Selam, a 3.3 million-year old fossil of a 3-year-old australopithecine infant from Ethiopia, exemplifies how paleoanthropologists can blur boundaries between living humans and apes.

Selam, if alive today, would not be confused with a three-year-old human—but neither would we mistake her for a living ape. Selam's chimpanzee-like hyoid bone suggests a more ape-like form of vocal communication, rather than human language capability. Overall, she would look chimp-like in many respects—until she walked past you on two feet. In addition, based on Selam's brain development, Alemseged theorizes that Selam and her contemporaries experienced a human-like extended childhood with a complex social organization.

Fast-forward to the time when Neanderthals lived, about 130,000 – 30,000 years ago, and most paleoanthropologists would agree that language capacity among the Neanderthals was far more human-like than ape-like; in the Neanderthal fossil record, hyoids and other possible evidence of language can be found. Moreover, paleogeneticist Svante Pääbo's groundbreaking research in molecular anthropology strongly suggests that Neanderthals interbred with modern humans. Paabo's work informs our genetic understanding of relationships to ancient hominins in ways that one could hardly imagine not long ago—by extracting and comparing DNA from fossils comprised largely of rock in the shape of bones and teeth—and emphasizes the great biological continuity we see, not only within our own species, but with other hominins sometimes classified as different species.

Though genetics has made truly astounding and vital contributions toward biological anthropology by this work, it's important to acknowledge the equally pivotal role paleoanthropology continues to play in its tandem effort to flesh out humanity's roots. Paleoanthropologists like Alemsegad draw on every available source of information to both physically reconstruct hominin bodies and, perhaps more importantly, develop our understanding of how they may have lived, communicated, sustained themselves, and interacted with their environment and with each other. The work of Pääbo and others in his field offers powerful affirmations of paleoanthropological studies that have long investigated the contributions of Neanderthals and other hominins to the lineage of modern humans. Importantly, without paleoanthropology, the continued discovery and recovery of fossil specimens to later undergo genetic analysis would be greatly diminished.

Molecular anthropology and paleoanthropology, though often at odds with each other in the past regarding modern human evolution, now seem to be working together to chip away at theories that portray Neanderthals as inferior offshoots of humanity. Molecular anthropologists and paleoanthropologists also concur that that human evolution did not occur in ladder-like form, with one species leading to the next. Instead, the fossil evidence clearly reveals an evolutionary bush, with numerous hominin species existing at the same time and interacting through migration, some leading to modern humans and others going extinct.

Molecular anthropologist Spencer Wells uses DNA analysis to understand how our biological diversity correlates with ancient migration patterns from Africa into other continents. The study of our genetic evolution reveals that as humans migrated from Africa to all continents of the globe, they developed biological and cultural adaptations that allowed for survival in a variety of new environments. One example is skin color. Biological anthropologist Nina Jablonski uses satellite data to investigate the evolution of skin color, an aspect of human biological variation carrying tremendous social consequences. Jablonski underscores the importance of trying to understand skin color as a single trait affected by natural selection with its own evolutionary history and pressures, not as a tool to grouping humans into artificial races.

For Pääbo, Wells, Jablonski and others, technology affords the chance to investigate our origins in exciting new ways, adding pieces into the human puzzle at a record pace. At the same time, our technologies may well be changing who we are as a species and propelling us into an era of "neo-evolution."

Increasingly over time, human adaptations have been less related to predators, resources, or natural disasters, and more related to environmental and social pressures produced by other humans. Indeed, biological anthropologists have no choice but to consider the cultural components related to human evolutionary changes over time. Hominins have been constructing their own niches for a very long time, and when we make significant changes (such as agricultural subsistence), we must adapt to those changes. Classic examples of this include increases in sickle-cell anemia in new malarial environments, and greater lactose tolerance in regions with a long history of dairy farming.

Today we can, in some ways, evolve ourselves. We can enact biological change through genetic engineering, which operates at an astonishing pace in comparison to natural selection. Medical ethicist Harvey Fineberg calls this "neo-evolution". Fineberg goes beyond asking who we are as a species, to ask who we want to become and what genes we want our offspring to inherit. Depending on one's point of view, the future he envisions is both tantalizing and frightening: to some, it shows the promise of science to eradicate genetic abnormalities, while for others it raises the specter of eugenics. It's also worth remembering that while we may have the potential to influence certain genetic predispositions, changes in genotypes do not guarantee the desired results. Environmental and social pressures like pollution, nutrition or discrimination can trigger "epigenetic" changes which can turn genes on or off, or make them less or more active. This is important to factor in as we consider possible medical benefits from efforts in self-directed evolution. We must also ask: In an era of human-engineered, rapid-rate neo-evolution, who decides what the new human blueprints should be?

Technology figures in our evolutionary future in other ways as well. According to anthropologist Amber Case, many of our modern technologies are changing us into cyborgs: our smart phones, tablets and other tools are "exogenous components" that afford us astonishing and unsettling capabilities. They allow us to travel instantly through time and space and to create second, "digital selves" that represent our "analog selves" and interact with others in virtual environments. This has psychological implications for our analog selves that worry Case: a loss of mental reflection, the "ambient intimacy" of knowing that we can connect to anyone we want to at any time, and the "panic architecture" of managing endless information across multiple devices in virtual and real-world environments.

Despite her concerns, Case believes that our technological future is essentially positive. She suggests that at a fundamental level, much of this technology is focused on the basic concerns all humans share: who am I, where and how do I fit in, what do others think of me, who can I trust, who should I fear? Indeed, I would argue that we've evolved to be obsessed with what other humans are thinking—to be mind-readers in a sense—in a way that most would agree is uniquely human. For even though a baboon can assess those baboons it fears and those it can dominate, it cannot say something to a second baboon about a third baboon in order to trick that baboon into telling a fourth baboon to gang up on a fifth baboon. I think Facebook is a brilliant example of tapping into our evolved human psychology. We can have friends we've never met and let them know who we think we are—while we hope they like us and we try to assess what they're actually thinking and if they can be trusted. It's as if technology has provided an online supply of an addictive drug for a social mind evolved to crave that specific stimulant!

Yet our heightened concern for fairness in reciprocal relationships, in combination with our elevated sense of empathy and compassion, have led to something far greater than online chats: humanism itself. As Jane Goodall notes, chimps and baboons cannot rally together to save themselves from extinction; instead, they must rely on what she references as the "indomitable human spirit" to lessen harm done to the planet and all the living things that share it. As Goodall and other TED speakers in this course ask: will we use our highly evolved capabilities to secure a better future for ourselves and other species?

I hope those reading this essay, watching the TED Talks, and further exploring evolutionary perspectives on what makes us human, will view the continuities and discontinuities of our species as cause for celebration and less discrimination. Our social dependency and our prosocial need to identify ourselves, our friends, and our foes make us human. As a species, we clearly have major relationship problems, ranging from personal to global scales. Yet whenever we expand our levels of compassion and understanding, whenever we increase our feelings of empathy across cultural and even species boundaries, we benefit individually and as a species.

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Evidence of Evolution


The Nobel Prize winning scientist Linus Pauling aptly described science as the search for truth.  Science does this by continuously comparing its theories objectively with evidence in the natural world.  When theories no longer conform to the evidence, they are modified or rejected in favor of new theories that do conform.  In other words, science constantly tries to prove its assumptions to be false and rejects implausible explanations.  In this way, scientific knowledge and understanding grow over time.  Religious explanations for the order of things are not science because they are based primarily on faith and do not subject themselves to be objectively falsified.  Because of this fundamental difference in the approach to understanding our natural world, the U.S. Supreme Court in effect decided in 1987 that the Biblically based "creation science" is not a science and cannot be taught as such in public schools as an alternative or in addition to the mainstream evolutionary theory of the biological sciences.  However, religious creation stories and the idea of "intelligent design" can be taught in philosophy, religion, or history courses.  Religion and Science provide different approaches to knowledge.  It is important to understand both.


What Is Evolution?

Biological evolution is genetic change in a population from one generation to another.  The speed and direction of change is variable with different species lines and at different times.  Continuous evolution over many generations can result in the development of new varieties and species.  Likewise, failure to evolve in response to environmental changes can, and often does, lead to extinction.

When scientists speak of evolution as a theory they do not mean that it is a mere speculation.  It is a theory in the same sense as the propositions that the earth is round rather than flat or that our bodies are made of atoms are theories.  Most people would consider such fundamental theories to be sufficiently tested by empirical evidence to conclude that they are indeed facts.  As a result of the massive amount of evidence for biological evolution accumulated over the last two centuries, we can safely conclude that evolution has occurred and continues to occur.  All life forms, including humans, evolved from earlier species, and all still living species of organisms continue to evolve today.  They are not unchanging end-products.

For those who have difficulty in accepting evolution because of what they perceive as contradictions with their fundamental religious beliefs, it may be useful to distinguish the ultimate origin of life from its later evolution.  Many, if not most, biological scientists accept that primordial life on earth began as a result of chance natural occurrences 3.5-4 billion years ago.  However, it is not necessary to believe in that view in order to accept that living creatures evolved by natural means after the origin of the first life.  Charles Darwin modified his religious beliefs, as did many others, as a result of the discovery of convincing proof of evolution.  Darwin's religious faith was also severely challenged by the death of his 10 year old daughter Annie in 1851.  Apparently, he came to believe that his God created the order of the universe including the rules of nature that result in biological evolution.  His famous book, On the Origin of Species, was not a denial of his God's existence.  However, he did reject a literal interpretation of the Judeo-Christian Bible.  His religious beliefs were probably very similar to those who advocate "theistic evolution" today.

  Isn't Evolution Just a Theory--video clip from PBS 2001 series Evolution.    This
       link takes you to an audio file at an external website.  To return here, you must click the
       "back" button on your browser program.           (length = 6 mins, 15 secs)
  Darwin's Personal Struggles--an interview with Darwin's biographer, James Moore.  This
       link takes you to an audio file at an external website.  To return here, you must click the
       "back" button on your browser program.           (length = 7 mins, 38 secs)
  Evolution of the Eye--an explanation by zoologist Dan-Erik Nilsson This link takes
       
you to a video at an external website.  To return here, you must click the "back"
        button on your browser program.                    (length = 4 mins, 8 secs) 

 
We now understand that there are a number of different natural processes that can cause evolution to occur.  These are presented in a later tutorial of this series (Modern Theories of Evolution).


How Do We Know That Evolution Has Occurred?

The evidence for evolution has primarily come from four sources:

1.    the fossil record of change in earlier species
2.the chemical and anatomical similarities of related life forms
3.the geographic distribution of related species
4.  the recorded genetic changes in living organisms over many generations


The Fossil Record

 
Geological strata containing an
 evolutionary sequence of fossils

Remains of animals and plants found in sedimentary rock deposits give us an indisputable record of past changes through vast periods of time.  This evidence attests to the fact that there has been a tremendous variety of living things.  Some extinct species had traits that were transitional between major groups of organisms.  Their existence confirms that species are not fixed but can evolve into other species over time.

The evidence also shows that what have appeared to be gaps in the fossil record are due to incomplete data collection.  The more that we learn about the evolution of specific species lines, the more that these so-called gaps or "missing links in the chain of evolution" are filled with transitional fossil specimens.  One of the first of these gaps to be filled was between small bipedal dinosaurs and birds.  Just two years after Darwin published On the Origin of Species, a 150-145 million year old fossil of Archaeopteryx was found in southern Germany.  It had jaws with teeth and a long bony tail like dinosaurs, broad wings and feathers like birds, and skeletal features of both.  This discovery verified the assumption that birds had reptilian ancestors.
 

  
Archaeopteryx fossil Archaeopteryx recreation Archaeopteryx tail feathers

Since the discovery of Archaeopteryx, there have been many other crucial evolutionary gaps filled in the fossil record.  Perhaps, the most important one, from our human perspective, was that between apes and our own species.  Since the 1920's, there have been literally hundreds of well-dated intermediate fossils found in Africa that were transitional species leading from apes to humans over the last 6-7 million years.  This evidence is presented in the last 3 tutorials of this series.
 

  Transitional Tetrapod Fossil--another example of filling what had been an evolutionary gap.
        This link takes you to a QuickTime video.  To return here, you must click the "back" button
        on your browser program.                     (length = 3 mins, 53 secs)

 

The fossil record also provides abundant evidence that the complex animals and plants of today were preceded by earlier simple ones.  In addition, it shows that multicelled organisms evolved only after the first single-celled ones.  This fits the predictions of evolutionary theory.


Chemical and Anatomical Similarities

Living things on earth are fundamentally similar in the way that their basic anatomical structures develop and in their chemical compositions.  No matter whether they are simple single-celled protozoa or highly complex organisms with billions of cells, they all begin as single cells that reproduce themselves by similar division processes.  After a limited life span, they also all grow old and die. 

All living things on earth share the ability to create complex molecules out of carbon and a few other elements.  In fact, 99% of the proteins, carbohydrates, fats, and other molecules of living things are made from only 6 of the 92 most common elements.  This is not a mere coincidence.

All plants and animals receive their specific characteristics from their parents by inheriting particular combinations of genes.  Molecular biologists have discovered that genes are, in fact, segments of DNA molecules in our cells.


section of a DNA molecule

These segments of DNA contain chemically coded recipes for creating proteins by linking together particular amino acids in specific sequences.


simple protein molecule
    

Human arm bones
(typical vertebrate pattern)

All of the tens of thousands of types of proteins in living things are mostly made of only 20 kinds of amino acids.  Despite the great diversity of life on our planet, the simple language of the DNA code is the same for all living things.  This is evidence of the fundamental molecular unity of life.

In addition to molecular similarities, most living things are alike in that they either get the energy needed for growth, repair, and reproduction directly from sunlight, by photosynthesis, or they get it indirectly by consuming green plants and other organisms that eat plants.

Many groups of species share the same types of body structures because they inherited them from a common ancestor that had them.  This is the case with the vertebrates, which are the animals that have internal skeletons.  The arms of humans, the forelegs of dogs and cats, the wings of birds, and the flippers of whales and seals all have the same types of bones (humerus, radius, and ulna) because they have retained these traits of their shared common ancient vertebrate ancestor.

All of these major chemical and anatomical similarities between living things can be most logically accounted for by assuming that they either share a common ancestry or they came into existence as a result of similar natural processes.  These facts make it difficult to accept a theory of special and independent creation of different species.


Geographic Distribution of Related Species

Another clue to patterns of past evolution is found in the natural geographic distribution of related species.  It is clear that major isolated land areas and island groups often evolved their own distinct plant and animal communities.  For instance, before humans arrived 60-40,000 years ago, Australia had more than 100 species of kangaroos, koalas, and other marsupials but none of the more advanced terrestrial placental mammals such as dogs, cats, bears, horses.  Land mammals were entirely absent from the even more isolated islands that make up Hawaii and New Zealand.  Each of these places had a great number of plant, insect, and bird species that were found nowhere else in the world.  The most likely explanation for the existence of Australia's, New Zealand's, and Hawaii's mostly unique biotic environments is that the life forms in these areas have been evolving in isolation from the rest of the world for millions of years.


Genetic Changes Over Generations

The earth's environments are constantly changing, usually in subtle and complex ways.  When the changes are so great as to go beyond what most members of a population of organisms can tolerate, widespread death occurs.  As Charles Darwin observed, however, not all individuals always perish.  Fortunately, natural populations have genetic diversity.  Those individuals whose characteristics allow them to survive an environmental crisis likely will be the only ones able to reproduce.   Subsequently, their traits will be more common in the next generation--evolution of the population will have occurred.

This process of natural selection resulting in evolution can be easily demonstrated over a 24 hour period in a laboratory Petri dish of bacteria living in a nutrient medium.  When a lethal dose of antibiotic is added, there will be a mass die-off.  However, a few of the bacteria usually are immune and survive.  The next generation is mostly immune because they have inherited immunity from the survivors.  That is the case with the purple bacteria in the Petri dishes shown below--the bacteria population has evolved. 

Evolution of antibiotic resistant bacteria 
 

This same phenomenon of bacteria evolution speeded up by human actions occurs in our own bodies at times when an antibiotic drug is unable to completely eliminate a bacterial infection.  That is the reason that medical doctors are sometimes hesitant to recommend an antibiotic for their patients and insist that the full dosage be used even if the symptoms of illness go away.  They do not want to allow any potentially antibiotic resistant bacteria to survive.

  Antibiotic resistance--how mutation and fast reproductive rates of
       microorganisms can outpace modern medical breakthroughsThis
       link takes you to an external website.  To return here, you must click
       the "back" button on your browser program. 
  
Dog variety resulting
from selective breeding
over many generations

People have developed many new varieties of plants and animals by selective breeding.  This process is similar to the bacteria experiment described above.  Selection of specimens to breed based on particular traits is, in effect, changing the environment for the population.  Those individuals lacking the desirable characteristics are not allowed to breed.  Therefore, the following generations more commonly have the desired traits.

 
 

Insect with a high
reproductive potential

Species that mature and reproduce large numbers in a short amount of time have a potential for very fast evolutionary changes.   Insects and microorganisms often evolve at such rapid rates that our actions to combat them quickly lose their effectiveness.  We must constantly develop new pesticides, antibiotics, and other measures in an ever escalating biological arms race with these creatures.   Unfortunately, there are a few kinds of insects and microbes that are now significantly or completely resistant to our counter measures, and some of these species are responsible for devastating crop losses and deadly diseases.

If evolution has occurred, there should be many anatomical similarities among varieties and species that have diverged from a common ancestor.  Those species with the most recent common ancestor should share the most traits.  For instance, the many anatomical similarities of wolves, dogs, and other members of the genus Canis are due to the fact that they are descended from the same ancient canine species and still share 99.8% of their genes.  Wolves and dogs also share similarities with foxes, indicating a slightly more distant ancestor with them.

  Genetic Tool Kit--evidence of a common set of genes for body parts shared by many, if not
        most, creaturesThis link takes you to a video at an external website.  To return, here, you
        must click the "back" button on your browser program.        (length = 4 mins, 47 secs) 

Given the abundant evidence supporting the theory of biological evolution, it is highly probable that evolution has occurred and is still occurring today.  However, there remains speculation in regards to the specific evolutionary path of some species lines and the relative importance of the different natural processes responsible for their evolution.

Much has been added to our understanding of the nature of evolution since the 19th century.  It is now known that there are six different processes that can operate independently or in consort to bring about evolution.  The understanding of these processes has become the basis for an overall synthetic theory of evolution.  This theory encompasses multiple causes, including Charles Darwin's concept of natural selection, Gregor Mendel's experimental results concerning genetic inheritance, as well as a number of crucial 20th century discoveries.  The synthetic theory of evolution will be revisited with more detail in the 6th tutorial of this biological anthropology series.


The Public Perception of Evolution in the United States

Biological evolution is far from being universally accepted by Americans.  Annual national polls carried out since the mid 1980's by the Center for Biomedical Communication at Northwestern University School of Medicine indicate that the percentage of Americans who accept evolution has dropped from 45% to 40%.  Curiously, the number who reject evolution have also dropped from 48% to 39% over the same time period.  Those who are uncertain about whether evolution occurs or not have increased from 7% to 21%.  While it is encouraging that fewer people are now hostile to the idea of biological evolution, the U.S. still has a higher percentage of its population who hold this view than 33 of the 34 European nations and Japan.  This is very likely a consequence of the relative emphasis placed on teaching science in public schools in the different countries.  In addition, anti-evolution sentiment is far stronger in American national politics.

For the vast majority of biologists, the debate over whether evolution occurs took place in the 19th century and has long been settled--evolution won.  The noted environmental biologist Theodosius Dobzhansky summed it up in 1973 by saying "nothing in biology makes sense except in the light of evolution".

  Evolving Ideas: Why Is Evolution Controversial Anyway--reconciling a belief in science and
        a religionThis link takes you to a video at an external website.  To return here, you must
        click the "back" button on your browser program.          (length = 6 mins, 36 secs) 

 


NOTE:   Some critics have said that the kinds of rapid evolutionary changes in insects and bacteria referred to above are not good evidence of the process of natural evolution because they occur as a result of human interference.  However, there is abundant evidence of rapid evolution occurring today independent of people.  An example was described by Cristina Sandoval in the May 23, 2002 issue of Nature.  A species of insect called the "walking stick" (Timena cristinae) found in the Santa Ynez Mountains of California now exists in two distinct varieties or forms that are in the process of evolving into two separate species by adapting to different environments.  The insect forms differ in terms of genetically determined color patterns--one is striped and the other is not.  The striped ones hide from predators on the striped chamise plant, while the unstriped ones hide on the unstriped blue lilac plant.  Those that have inherited the appropriate camouflaging color pattern for their chosen environment survive the onslaught of lizards and birds.  In this case, the natural predators, rather than humans, are the driving forces of natural selection.  Mating experiments show that each variety of "walking stick" prefers to mate only with others having the same color pattern.  This breeding isolation is leading to the evolution of two distinct species.

NEWS:   On July 17, 2005, The Pew Research Center for the People and the Press conducted a national poll in the United States concerning the teaching of creationism and evolution.  In regards to beliefs about how life developed, 42% of the respondents said that "living things have existed in their present form since the beginning of time."  Only 26% said that they had evolved through time as a result of "natural process such as natural selection," while 18% said that evolution occurred but was guided by a supreme being.  In response to the question of whether creationism should be taught in public schools instead of evolution, 38% said yes and 49% said no.  When asked whether creationism should be taught along with evolution, 64% said yes and 26% said not.  The older the respondent, the more likely he/she was to reject evolution and its teaching in favor of creationism.  The sample consisted of 2,000 people and the margin of error was �3.5%.

POSTSCRIPT:   For additional information regarding "creation science" and "intelligent design", look at the 2007 PBS Nova documentary and the entertaining 2008 follow-up lecture by Dr. Kenneth Miller linked below.

  Judgment Day: Intelligent Design on Trial--analysis of a controversial 2004 court
        case and its consequences.  This link takes you to a video at an external website.
        (length = 1 hr 55 mins)
  God, Darwin, and Design: Lessons from the Dover Monkey Trial--a 2008 lecture at
        the University of Texas by Kenneth Miller, a noted biologist.  This link takes you to
        a video at an external website.  You will be asked to first download the Envivio
        plugin in order to view the video.                 (length = 2 hrs 17 mins)

 


Copyright � 1998-2013 by Dennis O'Neil. All rights reserved.
illustration credits

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