I believe that our Heavenly Father invented man because he was disappointed in the monkey. I believe that whenever a human being, of even the highest intelligence and culture, delivers, an opinion upon a matter apart from his particular and especial line of interest, training and experience, it will always be an opinion so foolish and so valueless a sort that it can be depended upon to suggest to our Heavenly Father that the human being is another disappointment and that he is no considerable improvement upon the monkey.
Mark Twain, Autobiographical Dictation (1906)
With all the deference for the sensibilities of religious people, the idea that man was created in the image of God can surely be put aside.
Editorial, journal Nature, June 14, 2007
What makes us human? One way to shed light on this question is by comparing humans to other species to understand what makes them different. When Charles Darwin first proposed that humans descended from the great apes, he thought that the difference between humans and their closest relatives, the chimpanzees, was a quantitative difference, not a qualitative one. We were just fancier apes with bigger brains, Darwin reasoned. Was he right?
In the last years, scientists have made significant progress in understanding the genetic differences between humans and chimpanzees (Pan troglodytes). Of the three billion or so letters that make up the human genome, only about 35 million of them – about 1.2 percent – have changed in the six million years since the human and chimp lineages diverged. This means that somewhere among those roughly 35 million DNA base pairs lay the differences that make us human.[1]
Millions of bases are a vast territory to search. So far, scientists have been able to identify just a few areas in that DNA segment that have experienced rapid change since humans and chimps split from a common ancestor. Normally, genetic mutations accumulate at a steady rate, what scientists refer to as the “ticking of the molecular clock.” In contrast, acceleration in that rate of change in some part of the genome is a hallmark of positive selection, in which mutations that help an organism survive and reproduce are more likely to be passed on to future generations. In other words, those parts of the code that have undergone the most modification since the chimp-human split are the sequences that most likely shaped humankind.
One area in the genome that has experienced accelerated change is known, appropriately, as Human Accelerated Region 1 (HAR1). It turns out that until humans came along, HAR1 evolved extremely slowly. In chickens and chimps – whose lineages diverged some 300 million years ago – only two of the 118 bases differ, compared with 18 differences between humans and chimps, whose lineages diverged far more recently. The fact that HAR1 was essentially frozen in time through millions of years indicates that it plays a critical role or function; that it then underwent abrupt revision in humans suggests that this function was significantly modified in our lineage.
Studies reveal that HAR1 is active in a type of neuron that plays a key role in the pattern and layout of the developing cerebral cortex, the wrinkled outermost brain layer. When things go wrong in these neurons, the result may be a severe, often deadly, congenital disorder known as lissencephaly (“smooth brain”), in which the cortex lacks its characteristic folds and exhibits a markedly reduced surface area. Malfunctions in these same neurons are also linked to the onset of schizophrenia in adulthood.[2]
The FOXP2 gene is another fast-changing sequence identified by scientists. It is estimated that the modern version of the FOXP2 gene emerged at least half a million years ago. As we saw during our discussion of evolution (see also Box 4), people with mutations in this gene are unable to make certain subtle, high-speed facial movements needed for normal human speech, even though they possess the cognitive ability to process language.
Language is a key characteristic that sets us apart from chimpanzees. Most of what distinguishes human language from vocal communication in other species, comes not from physical means but cognitive ability, which is often correlated with brain size. Primates generally have a larger brain than would be expected from their body size. But human brain volume has more than tripled since the chimp-human ancestor – a growth spurt that researchers have only begun to unravel. Two genes, we also saw, microcephalin and ASPM, underwent accelerated evolution at times after the chimp and human lines diverged, hinting they played a role in the emergence of our impressive brain size. When these genes malfunction, the result is a condition called microcephaly in which brain size severely diminishes.
Box 4: The Genes that Make Us Human
The analysis of variation in the human genome suggests that human evolution accelerated enormously in the last 40,000 years under the force of natural selection. In fact, the evolution of our gray matter is still ongoing. Recent research has focused on two genes called microcephalin and ASPM. When the genes malfunction, the result is a condition called primary microcephaly in which brain size severely diminishes. Previous studies across millions of years of primate lineages indicated that these genes underwent accelerated evolution at times after the chimp and human lines diverged, hinting they played a role in the emergence of our impressive brain size.1
What is amazing is the significant impact that small variations in the genotype can have in the phenotype (i.e., the organism). We now know, for example, that the human version of a gene called FOXP2, which plays a role in our ability to develop speech and language, evolved within the last 200,000 years, the period in which anatomically modern humans first appeared. By comparing the protein coded by the human FOXP2 gene with the same protein in various apes and in mice, researchers discovered that the amino-acid sequence that makes up the human variant differs from that of the chimpanzee in just two locations out of a total of 715. Incredibly, this small change explains the emergence of all aspects of human speech. Indeed, humans born with a defective FOXP2 gene have trouble articulating words and understanding grammar.2
At the same time, “old fashioned” field observations and excavations have shed new light on how populations diverge to form new species (known as macroevolution). New species can form when populations of an existing species begin to adapt in different ways and eventually stop interbreeding. This can happen when populations wind up on opposite sides of oceans or mountain ranges. Sometimes, however, a single, contiguous population splits into two. The theory of evolution predicts that this splitting begins when some individuals in a population stop mating with others. Field biologists have recorded compelling examples of that process, some of which featured surprisingly rapid evolution in organisms’ shape and behavior.3 In Idea 16 I will examine other genes that contributed to our humanness.
1. Apparently, the microcephalin variant arose about 37,000 years ago and the ASPM one just 5,800 years ago. These findings provide evidence that the human brain is evolutionarily plastic and may still be evolving. See, e.g., Balter, M. (2005). Are human brains still evolving? Brain genes show signs of selection, Science, 309, 1662-1663.
2. Enard, W., et al. (2002). Molecular evolution of FOXP2, a gene involved in speech and language. Nature, 418: 869-872.
3. Culotta, E. & Pennisi, E. (2005). Breakthrough of the year: Evolution in action, Science, 310, 1878 – 1879.
The inter-species study of the genome reveals other surprises. It turns out that where DNA substitutions occur in the genome – rather than how many changes arise overall – can matter a great deal. In other words, you do not need to change much of the genome to make a new species. “The way to evolve a human from a chimp-human ancestor is not to speed the ticking of the molecular clock as a whole. Rather the secret is to have rapid change occur in sites where those changes make an important difference in an organism’s functioning.”[3]
HAR1 is certainly such a place. So, too, are the FOXP2, microcephalin and ASPM genes, which contain another of the fast-changing sequences. Scientists have also found several other places in the genome whose rapid change could have contributed to our humanness.[4]
The cognitive skills that make us human
Undoubtedly, comparative studies of the genome have greatly increased our understanding of what makes humans, well, human. Hopefully, we will continue to learn more as scientists explore the rest of the genome for clues as to how humans diverged from other species. Equally important, though, is to understand how these genetic changes translate into cognitive skills that are uniquely human. Human and chimpanzee infants share many similarities during the first year. Both species locomote (move from place to place), show interest in unexpected or unfamiliar events and remember where an attractive object disappeared ten seconds earlier.
In fact, some scientists remain resistant to acknowledging that any human quality is unique. As the late Harvard University psychologist Jerome Kagan writes, “When a linguist claims that only humans have a language with a grammar, a scientist will reply that chimpanzees can be taught to communicate with pieces of plastic.” [5] However, as research by Kagan and others show, just two years later, children have diverged permanently from their primate relatives. In particular, the human brain experiences much more rapid establishment of connectivity during this critical period. This added connectivity of brain cells, so it seems, sets us on the course to those things that make us uniquely human, such as language acquisition and cultural knowledge.[6]
Let’s look at seven of the key features that humans display, as compared to chimpanzees, after the 24-month point:[7]
1. The ability to infer intentions, evaluations, and feelings of others. This ability is evident in an experiment where an adult hides a toy under one of three covers behind a barrier so that the child cannot see where the toy is hidden. If, after removing the barrier, the adult directs her gaze toward the toy’s location, two-year olds reach in the direction of the adult’s gaze, indicating that they assume the adult is looking at the place where the toy rests. Such an ability is understood to be necessary for feeling empathy with another. While chimpanzees occasionally track the gaze of another animal, they apparently do not understand that another animal intends to share information with them.
2. Use of language to represent abstract ideas. While apes can be taught, with considerable training, to treat pieces of plastic as symbolic of objects, no chimpanzee comes close to the linguistic ability of the average four-year-old who uses language to represent abstract ideas.
3. Morality. Every human society has semantic concepts for the ideas of good, bad, right, wrong, and most humans experience a changed feeling when they contemplate, or commit, a behavior that violates a standard they regard as proper. While chimpanzee behavior shows that they possess rules and punish those who break them, they do not necessarily seem to experience “guilt”. Also, apes do not seem to show signs of anger upon seeing an unfamiliar animal take food from another, provided the victim was not a blood relative.
4. An acute consciousness of one’s feelings, intentions, and properties. Humans seem to experience four different forms of consciousness or self-awareness. One category is the moment-to-moment changes in sensations that originate in, for example, the taste of chocolate. A second form is consciousness of one’s physical features, beliefs, talents, personality traits, moods, and social categories. The third form is the awareness that one is about to implement or suppress an action. A fourth form is the interpretation of one’s feelings, perceptions, and action through the construction of a coherent explanation of the state of consciousness at a given time. Chimpanzees might be aware of the taste of some fruits, and of the patterns of light and shadow on the forest floor, but at this point there is no evidence they possess forms of consciousness beyond the sensory (although see Idea 15).
5. Remember the past and anticipate the future. Although apes can remember the past and anticipate the future, humans expand both timelines to distant horizons. Sixty-year-olds can remember their first day of school and can anticipate what might occur two decades in the future when senility is imminent. There is no evidence that chimpanzees can anticipate or recollect events decades before or after the present moment (although scientists may need to find better ways to “ask” this question).
6. Attraction to new experiences. The human attraction to new experiences does expands to primates. Chimpanzees seek new fruits to eat, new places to rest, and new mating partners. However, humans spend more time than any other animal looking for unfamiliar events that can be comprehended and new skills that can be mastered (hence you’re reading this book!).
7. Ability to dissociate survival to reproductive maturity from inclusive fitness. The biological concept of inclusive fitness is a relative property. It is defined by the reproductive success of each person and all her genetic relatives, compared with the success of a related species in the same ecological setting. The invention of effective contraceptives has allowed humans to limit the size of their family or, in some cases, to have no children at all. This decision is inconsistent with the biological demand to maximize inclusive fitness.
Admittedly, some of the differences between humans and chimpanzees discussed here remain controversial and further research may provide a more nuanced description. The list may change and/or be modified as scientists gain more knowledge. However, it is undeniable that deep differences exist between the species, and it is important that we understand what these are.
To conclude: Treasure the traits that make you human
We should all treasure and nurture the traits that make us human, such as the ability to communicate, empathy, morality, self-awareness, and the ability to remember the past and anticipate the future, which are important factors in our discussion of free will (Idea 15, forthcoming).
Of all the seven traits discussed above that make us uniquely human, the one that I believe captures the essence of humanness is the last one on the list: We can deviate from the “program” that biology and evolution has laid out for us. That is, humans do not blindly follow the commands of the “selfish-gene” to maximize inclusive fitness. We are not just fancier apes with bigger brains, as Darwin thought.
Certainly, most humans are driven by strong instincts to have sex and reproduce. Witness the exponential growth of the human population and its ability to dominate most habitats on Earth. However, we can also decide to remain childless and act against our own survival instinct. This presents opportunities, but also risks, for the human species. In effect, either we will thrive and explore other worlds and maybe even other universes, or go extinct, just like the dinosaurs. I will revisit some of these issues in Idea 18 (forthcoming).
[1]The Chimpanzee Sequencing and Analysis Consortium (2005). Initial sequence of the chimpanzee genome and comparison with the human genome. Nature, 437: 69-87. When DNA insertions and deletions are taken into account, however, humans and chimps share only about 96 percent of their sequence. See also http://www.genome.gov/15515096. [2]K.S. Pollard (2009). What makes us human? Scientific American, May: 44-49 [3]Ibid, p.48. [4]See e.g., “Searching for the Genes That Are Unique to Humans,” by Ed Yong. The Atlantic, Oct 13, 2015. [5]Kagan, J. (2004). The uniquely human in human nature, Dædalus, Fall, pp 77. [6] Sakai, T. et al. (2012). Developmental patterns of chimpanzee cerebral tissues provide important clues for understanding the remarkable enlargement of the human brain Proc. R. Soc. B. (published Dec 19, 2012). [7]Kagan, J. (2004). The uniquely human in human nature, Dædalus, Fall:77-88.
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