Deirdre Joy (

National Institutes of Health/Infectious Diseases

Southeastern Medieval Association, October 2006

BABEL Panel: Premodern to Modern Humanisms

The Planet of the Apes Comes to Mind

What does it mean to be human?  What are the biological and genetic traits that define and distinguish us? When these distinctions are blurred or erased what does it then mean to be human? Our closest relative is the chimpanzee (Pan troglodytes).

Latin troglodytae: cave dwelling people

  1. A member of a supposed prehistoric race that lived in caves or holes
  2. (by extension) Anything that lives underground
  3. A reclusive, reactionary or out-of-date person, especially if brutish
  4. (computing) A person who chooses not to keep up-to-date with the latest software and hardware

Our two lineages probably split approximately five million years ago, though the date gets revised forward with each new fossil discovery. Humans did not evolve from chimpanzees. They are not our ancestors. It is not true that we were once chimps, though that is exactly what I told my six-year-old daughter for simplicity when she asked, “Who was the first human?” Pan troglodytes is not our ancestor, but rather our sister, as we share a common, though long extinct, ancestor, from which our two branches diverged, and probably are continuing to diverge. And yet, it’s hard to think of chimpanzees as our sisters. Ancestors, okay, that was after all a long time ago, we’ve come a long way since then. But sisters?  Michael Pollan in The Omnivore's Dilemma, addresses the need to distance ourselves from this connection when he states, “So much of the human project is concerned with distinguishing ourselves from beasts that we seem strenuously to avoid things that remind us that we are beasts too. Exactly why we would strive so hard to distance ourselves from our animality is a large question, but surely the human fear of death figures in the answer. One of the main thoughts about animal death is: will my own death be like animals or not?”

But, okay, it is true that a lot has happened during the long strange trip out to the tip of our particular evolutionary branch on the Tree of Life, and that many of the changes that have occurred define us as humans, as opposed to, say chimps. Some of the biological traits that distinguish us from other primates are a larger brain, smaller canines, upright walking, elaborate language, reduced hair cover, more efficient thermo-regulation through sweating. We’re better long-distance runners too. But it’s the larger brain, specifically the enlarged neocortex, more than any other innovation that has defined the distinctive evolutionary niche we find ourselves in – that is to say we are everywhere – there is no place on the planet our presence and our impact isn’t felt. But still, evolution of larger brains is not uniquely human. Brains were getting larger in primates long before humans appeared (30-40 million years ago in New and Old World monkeys, and again 8-19 million years ago in modern hominids). Though the largest neocortical increase occurred over the past three million years in the human lineage. And it does seem obvious to me that the human brain has abilities, whether in kind or degree or both, that are distinct and unmatched in nature.

Many scientists are currently in pursuit of the genetic basis of what makes us human. Large neocortex size is heritable and so must be encoded in some way in our genes. Referred to as human lineage specific (HLS) genomic changes. A little over a year ago, during the same week that Hurricane Katrina struck, a first draft of the chimpanzee genome was completed. Geneticists have since scoured the chimp and human genomes for features that are uniquely human. The question driving much of this research is: what is it that sets us, as humans, apart from our closest animal relative? What are the “humanness” genes? Although with only two completed genomes – human and chimp – you can’t actually assign a feature as uniquely human. It could be unique to the chimp instead. If we could make comparisons with other primate genomes we might find that a particular feature is in fact unique to the chimp (or gorilla) and shared by other higher primates including humans.

Human and chimp DNA differ by only 1.23%. It doesn’t sound like a lot but there are billions of letters – As, Ts, Cs, and Gs, known as base pairs - in the human genome, so a 1% difference amounts to about 1 mutation per 100 base pairs, which, given billions of base pairs, actually ends up being a substantial number of differences. In addition, structural variations, including copy number differences, insertions and deletions, and inversions, and changes in non-protein coding DNA, constitute a significant source of genomic variation between chimps and humans (raising the level of divergence to ~3%). And, a small genetic change can lead to a large phenotypic difference

There are about 27 genes currently known that show human lineage specific (HLS) changes, either in terms of mutations or copy number, and that relate to cognition or are otherwise brain related. Mutations in some are implicated in language deficits in humans, changes in brain size, increases in energy demands of the brain. Examples:


ASPM are the most common known cause of MCPH, a neurodevelopment disorder characterized by a reduction in cerebral cortical volume down to about the size of that of early hominids 2. Mutations in the fruit fly homolog – asp – arrest the division of progenitor nerve cells (neuroblasts). It appears that selection on ASPM occurred well before human brain expansion, suggesting that the originally selected function of ASPM was for something other than large brain size.

2. FOXP2

 Mutations in the human FOXP2 gene lead to difficulty articulating and understanding language.

3. Extreme genome duplication of DUF1220 domains in the human lineage

Humans have 212 repeats of the DUF1220 domain, compared with 37 repeats in African great apes, 30 in orangutan and Old World monkeys, a single-copy in non-primate mammals, and absent in non-mammals.

These are a few examples of genes that may (or may not) set us apart from our non-human counterparts. In the field of stem cell research, some scientists are working in the opposite direction: In 2001 a paper was published in Science in which human brain stem cells were grafted into the brains of fetal monkeys. Subsequently, a working group was convened to consider the ethical implications of such studies and the findings were published in Science in 2005. The working group unanimously rejected ethical objections grounded on unnaturalness or crossing species boundaries. A recent National Academy report notes the notion that there are fixed species boundaries is not well supported in science or philosophy. This was news to me.

They agreed that the central issue is whether introducing human cells into NHP brains raises questions about moral status. One conceivable result of H-HNP neural grafting is that the resulting creature will develop humanlike cognitive capacities relevant to moral status. How does that change our moral obligations towards them? To the extent that a NHP attains those capacities through neural grafting, that creature must be held in correspondingly high moral standing. (Which I suppose might include not having to undergo neural grafting).

The criteria deemed plausible and widely accepted for determining moral status were mental capacity such as the ability to feel pleasure and pain, language, rationality, richness of relationships. But there’s a problem here, acknowledged by the working group - establishing whether and in what ways engrafted animals undergo cognitive or behavioral changes requires and understanding of what the normal range is for a particular NHP species. But we really don’t know. The report concedes that, “Even if we observe what appears to be more humanlike capacities in an engrafted animal, we may be unable either to establish whether capacities are outside of the normal range of that species or to interpret the moral meaning of observed changes.”

Some in the working group even argued that such changes might constitute a potential benefit to the engrafted animal, insofar as the changes are viewed as enhancements of the sort we value for ourselves. Of course, these more humanlike capacities might also confer greater capacity for suffering. The Planet of the Apes comes to mind. Or as Michael Pollan states in The Omnivore's Dilemma, “Human pain differs from animal pain by an order of magnitude, largely due to our possession of language and, by virtue of language, our ability to have thoughts about thoughts and to imagine what is not. Suffering depends on a degree of self-consciousness only a handful of animals appear to command. Suffering as pain amplified by distinctly human emotions such as regret, self-pity, shame, humiliation, and dread.”

So, to return to my original question—what does it mean to be human—we might also ask ourselves, how is it possible to distinguish between, say, human and animal suffering, when our only perspective on this question is the so-called “human” one? To be human gives us the capacity to be able to ask these important questions, but also limits our power to conceptualize the answers from the point of view of the animal consciousness. But this also raises the even more important question: why do we strive so hard to distinguish ourselves from animals in the first place? As science has shown, we are not the descendants of higher primates—having supposedly evolved from our former animal selves—but rather, are related to them as siblings. And even as we search for the genetic underpinnings of our differences with one hand, we are creating technologies that have the potential to erase them with the other. Why then, in our debates over our moral obligations towards animals, do we always invoke the rhetoric of hierarchy, stewardship, and dominion, over the rhetoric of family and communion?

Works Cited

Pollan, M. The Omnivore's Dilemma: A Natural History of Four Meals (The Penguin Press, 2006).

Evans, P., Anderson, J., Vallender, E., Gilbert, S. & Malcolm, C. "Adaptive evolution of ASPM, a major determinant of cerebral cortex size in humans." Human Molecular Genetics 13, 489-494 (2004).

Lai, C., Fisher, S., Hurst, J., Vargha-Khadem, F. & Monaco, A. "A forkhead-domain gene is mutated in a severe speech and language disorder." Nature 413, 519-523 (2001).

Popesco, M. et al. "Human lineage-specific amplifiction, selection, and neuronal expression of DUF1220 domains." Science 313, 1304-1307 (2006).

Ourednik, V. et al. "Segregation of Human Neural Stem Cells in the Developing Primate Forebrain." Science 293, 1820-1824 (2001).

Greene, M. et al. "Moral Issues of Human-Non-Human Primate Neural Grafting." Science 309, 385-386 (2005).