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Defining Chimeras ... and Chimeric Concerns
Henry T. Greely
Chimera, n.... 1. a. A fabled fire-breathing monster of Greek mythology, with a lion's head, a goat's body, and a serpent's tail (or according to others with the heads of a lion, a goat, and a serpent), killed by Bellerophon.
Oxford English Dictionary (1989)
The original chimera turns out to be surprisingly undefined. Did Bellerophon, riding Pegasus, slay a monster with the heads of three different species or a one-headed beast with parts from three species? This lack of clear definition exists in contemporary discussions of the ethics of nonmythological chimeras, including in the useful article by Jason Scott Robert and Françoise Baylis, "Crossing Species Boundaries" (2003). In their third paragraph Robert and Baylis list a broad set of possible types of chimeras before, in their fourth paragraph, focusing on human-to-animal embryonic chimeras. I believe that we can achieve a better understanding of the ethical issues raised by chimeras—and, indeed, whether the category "chimera" is useful in ethical discussion of contemporary biology—by defin- ing chimera more exhaustively and then examining the concerns associated with different types of chimeras. In this commentary, therefore, I first offer a taxonomy of chimeras and then speculate on how that taxonomy might illuminate the ethical issues the category raises. I conclude that ethical issues are not raised by whether something is a chimera but on the basis of three other questions about the chimeric organism: its "humanity," its "naturalness," and its proposed uses.
The Oxford English Dictionary (OED) did not give a biological definition of chimera in its 1971 edition, although the famous second edition of Webster's Unabridged Dictionary had already provided a botanical definition by 1934:
"4. Bot. A mixture of tissues of different genetic constitution in the same part of a plant."
In its 1989 second edition the OED added, as the fourth figurative definition, the following:
"d. Biol.... An organism (commonly a plant) in which tissues of genetically different constitution co-exist as a result of grafting, mutation, or some other process."
The OED traced the term to a German scientist, H.Winkler, in 1907.
Robert and Baylis go beyond all of these definitions, not only in the specifics of some of their proposed chimeras, unanticipated by dictionary writers, but by including hybrids. I believe their broader approach is appropriate. The core idea in the biological use of chimera is captured by the following broad definition: "a single biological entity that is composed of a mixing of materials from two or more different organisms." This broad definition can then be played out across four important dimensions:
- the biological constituents that are mixed;
- the relationship between the two organisms being mixed;
- how the mixing is done ("naturally" or "unnaturally," by which, for this purpose, I mean through technical human intervention); and
- when the mixing takes place.[End Page 17]
The biological constituents mixed vary substantially. In current uses they can be cells, tissues, or larger body parts. They can be gametes (eggs and sperm), as in hybrids. Or they can be genes—individually, collectively, or, via nuclear transfer, as whole genomes.
The organisms can be related in various ways. They might be from the same "type," "breed," or "race" of thesame species. They might be from different subparts of the same species. They might be from different but closely related species. Or they might be from distantly related species.
Some forms of "mixing" occur naturally, as the mixing of egg and sperm produces, in sexually reproducing species, a single biological entity that is composed of a mixing of materials from two different organisms. Others can be done only with human intervention using relatively nontechnical (and often long-practiced) methods, such as the common practice of grafting limbs from one species of fruit tree onto the trunk of a different species. Still other forms of mixing, such as the creation of embryonic chimeras across species lines, require technical (and relatively new) human intervention...