Monday, April 1, 2013

Empedocles, Lucretius and lateral gene transfer


Empedocles (c. 490–430 BCE) and Lucretius (c. 99-55 BCE) have been credited with first articulating the theory of "survival of the fittest" (Sedgley 2003). However, this is of interest only to Darwinian scholars, who focus solely on trees. What is of more interest to scholars of phylogenetic networks is that these same two philosophers have also been credited with first suggesting the doctrine of horizontal gene transfer (Wilkins 2009). Gene transfer is, of course, an important source of reticulate evolution.

Empedocles was a Greek philosopher, a citizen of what is now Agrigento, in Sicily. He is perhaps most famous for first outlining the elemental theory of the physical world (ie. Air, Earth, Fire, Water). Moreover, he identified two fundamental forces, which he called love and strife. Love is the force that brings objects together, while Strife is the force that drives them apart. Empedocles postulated that the universe was once condensed into a tight sphere by the force of love, and strife later exploded this into an expanding mass. This has been seen as a forerunner of modern ideas about the Big Bang and the subsequent expanding universe.

More importantly for our purposes, Empedocles had a physical theory about the random development of living forms. According to this theory, Life first emerged as a collection of disassociated body parts, which wandered about on their own, without the intervention of divine power. These were not parts severed from previously complex beings, but each functioned in its own right as an independent "single-limbed" being. Complex creatures were then created by the accidental combination of these disparate limbs and organs. If the correct parts combined, then the creature would survive and go on to found a species, but if the wrong combination occurred then the creature would perish — only those with the most suitable combinations survived, by a process that we now call natural selection.

Empedocles' hypothesized hybrid creatures were literally mocked by later Greek philosophers, notably Aristoteles (384-322 BCE) and Epicurus (341-270 BCE), and their followers. They derided these monsters as "roll-walking creatures with hands not properly articulated or distinguishable" and as "ox-headed man-creatures". It was Lucretius who resurrected Empedocles' idea, in his only known work (De Rerum Natura), which was about the beliefs of Epicureanism — Lucretius was the first writer to introduce Roman readers to Epicurean philosophy.

Titus Lucretius Carus was a Roman poet and philosopher, apparently resident in Rome itself. He is perhaps most famous for his atomistic view of the physical world (everything is built up from collections of indivisible particles). More importantly for our purposes, Lucretius expounded a similar theory to that of Empedocles, namely that originally a set of randomly composed monsters sprang up, of which only the fittest survived. However, whereas Empedocles described isolated limbs as the starting point, Lucretius described whole organisms with defective combinations of body parts (what we would now call congenital defects), so that his maladapted creatures were formed at the atomic level rather than at the macroscopic level of whole limbs. Also, in Lucretius' theory there was apparently no inter-species mingling of limbs, as there was in Empedocles' version.

These two related theories of zoogony appear to have lain dormant for a couple of thousand years, crushed under the iron fist of Aristoteleanism. Even into the 1900s, biology could be best described as being essentially an extension of Aristoteles' philosophical ideas (Mayr 1982). Nevertheless, slowly the idea of natural selection was re-introduced to biology, notably with the work of √Čtienne Geoffroy Saint-Hilaire (1772-1844), and culminating in the work Alfred Russel Wallace (1823-1913) and Charles Robert Darwin (1809-1882).

However, even after the introduction of this evolutionary idea, the focus was on the inheritance of morphological modifications, not on the admixture of parts inherited from different organisms; and so only half of Empedocles' ideas were accepted.

It took until the dawn of the 20th century for the Russian lichenologist Constantin Sergeevich Mereschkowsky (1855-1921) to first outline a cellular version of Empedocles' vision. It had recently been shown that lichens involve a symbiotic relationship between fungi and algae, very much along the lines first envisioned more than 2,200 years before. Mereschkowsky extended this idea to the sub-cellular level, with the explicit goal of explaining the evolutionary development of land plants from algae-like forms of life, postulating that chloroplasts originated as symbiotic blue-green algae. The German histologist Richard Altman (1852-1900) had already hinted that what we now call mitochondria (he called them bioblasts) are bacterial symbionts. It was some time later that the American anatomist Ivan Emanuel Wallin (1883-1969) published Symbionticism and the Origin of Species, in which he explicitly suggested that symbiotic bacteria have played a fundamental role in the evolution of species.

This development culminated in the suggestion that genes themselves can be transferred between distant organisms, thus bringing thought down to the atomistic level envisioned by Lucretius. This revealed the hybrid nature of many genomes, even in situations where phenotypic admixture is not manifest. The first description of horizontal gene transfer is usually credited to Victor J. Freeman (in 1951), who demonstrated that the transfer of a viral gene into a bacterium could create a virulent strain from a non-virulent strain. Since then, lateral gene transfer has been widely reported as an important component of prokaryote evolution; and it has increasingly been reported in eukaryotes as well.

We have thus come full circle. Empedocles first introduced the theory of "survival of the fittest", which took nearly 2,300 years to be re-discovered by science, as well as outlining the basic concept of "horizontal gene transfer", which took an extra century for its renaissance.

All of the information presented here is factually correct. However, only on All Fool's Day can the facts be combined in this outrageous way, and such a history be told with a straight face.

References

Mayr E. (1982) The Growth of Biological Thought: Diversity, Evolution and Inheritance. Belknap Press, Cambridge MA.

Sedgley D. (2003) Lucretius and the new Empedocles. Leeds International Classical Studies 2.4.

Wilkins J.S. (2009) New work on lateral transfer shows that Darwin was wrong. ScienceBlogs Evolving Thoughts March 31 2009.

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