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Peter Medawar (1915–1987)

Medawar was an immunologist and biologist whose career produced two distinct bodies of work, each influential in its own domain. The first was transplantation immunology: his experimental demonstration of acquired immunological tolerance — that the immune system can be trained during development to accept foreign tissue — won the Nobel Prize in Physiology or Medicine (1960, shared with Frank Macfarlane Burnet) and opened the path to organ transplantation. The second was the evolutionary biology of ageing: his 1952 lecture “An Unsolved Problem of Biology” proposed the mutation accumulation theory of senescence, one of the two foundational evolutionary accounts of why organisms age. He was also a prolific essayist on the philosophy and practice of science, widely regarded as one of the finest scientific prose stylists of his generation.

Life

Born 28 February 1915 in Petrópolis, Brazil, to a Lebanese father (a businessman) and a British mother. The family moved to England when Medawar was young. Educated at Marlborough College and Magdalen College, Oxford, where he studied zoology under J. Z. Young. DPhil from Oxford (1941), working on tissue culture and the biology of wound healing — research that began during the Second World War, when he was asked to investigate why skin grafts from donors were rejected by burn patients.

Fellow and lecturer at Magdalen College, Oxford (1938–47). Mason Professor of Zoology at the University of Birmingham (1947–51). Jodrell Professor of Zoology and Comparative Anatomy at University College London (1951–62). Director of the National Institute for Medical Research at Mill Hill, London (1962–71).

Suffered a severe cerebral haemorrhage in 1969 while reading the lesson at the British Association meeting in Exeter — a stroke that left him partially paralysed. He continued working despite the disability for the remaining eighteen years of his life. Nobel Prize in Physiology or Medicine (1960). Knighted (1965). Order of Merit (1981). Fellow of the Royal Society (1949). Died 2 October 1987, aged seventy-two.

Immunological tolerance

The problem Medawar addressed: why does the body reject transplanted tissue from other individuals? His wartime work on burn patients showed that skin grafts from donors were destroyed by an immune response — and that second grafts from the same donor were rejected faster than the first, indicating immunological memory. The rejection was specific, learned, and cumulative.

The breakthrough came from a prediction by Frank Macfarlane Burnet and Frank Fenner (The Production of Antibodies, 1949): if the immune system learns during embryonic development to distinguish self from non-self, then exposing an embryo to foreign tissue before the immune system matures should teach it to accept that tissue as self. Medawar and his colleagues (Rupert Billingham and Leslie Brent) tested this experimentally. They injected cells from one strain of mouse into embryos of another strain. When the injected mice matured, they accepted skin grafts from the donor strain that would normally have been rejected — acquired immunological tolerance.

The 1953 paper (Billingham, Brent, and Medawar, “Actively acquired tolerance of foreign cells,” Nature 172) demonstrated that immune tolerance is not hardwired but learned during development, and that the learning can be manipulated. The result confirmed the clonal selection theory (later developed fully by Burnet) and established the immunological foundation for organ transplantation. The Nobel committee cited it as “the gruesome gift that made transplantation possible.”

The evolution of ageing

“An Unsolved Problem of Biology” (1952), an inaugural lecture delivered at University College London and published as a pamphlet. Medawar asked: why do organisms age? Natural selection should, in principle, favour indefinite survival — a longer-lived organism has more opportunities to reproduce. Yet ageing is universal among complex organisms.

Medawar’s answer: the force of natural selection declines with age. In any population subject to external mortality (predation, disease, accident), fewer individuals survive to older ages regardless of their intrinsic condition. A gene with harmful effects that manifest only late in life faces weak selection against it — most of its carriers have already reproduced, and many are already dead from other causes. Such genes accumulate in the genome because selection cannot efficiently remove them. Ageing is the cumulative result of this mutation accumulation.

George C. Williams’ antagonistic pleiotropy hypothesis (1957) proposed a complementary mechanism: that selection actively favours genes with early benefits and late costs. Where Medawar’s theory is about what selection fails to remove, Williams’ is about what selection actively promotes. Together, the two theories form the twin pillars of the evolutionary biology of ageing. They are not mutually exclusive: mutation accumulation and antagonistic pleiotropy may both contribute, and distinguishing their relative importance in particular organisms is an active area of research.

Writing on science

Medawar was an essayist of unusual range and precision. The Art of the Soluble (1967) and Pluto’s Republic (1982) collected essays on the practice and philosophy of science. Advice to a Young Scientist (1979) was a practical and philosophical guide. His book reviews — particularly the review of Teilhard de Chardin’s The Phenomenon of Man (1961), which he described as “the greatest work of bad science I have ever read” — are remembered for their clarity and sharpness.

His philosophical position was broadly Popperian: science advances by conjecture and refutation, not by induction from observation. The Limits of Science (1984) explored what science can and cannot address. He was widely regarded as one of the finest scientific prose stylists of his generation; Dawkins dedicated The Extended Phenotype to him.

Where Medawar stops

Medawar’s evolutionary theory of ageing establishes the principle — the declining force of selection with age — but does not develop the population-genetic or molecular detail. The principle explains why ageing can exist; it does not predict the rate of ageing, the specific mechanisms of senescent decline, or how ageing evolves differently across species with different life histories. Williams’ antagonistic pleiotropy provided a sharper mechanism; subsequent work in biogerontology (Michael Rose, Linda Partridge, and others) has built the experimental and comparative programme that tests both theories against data. Medawar supplied the foundational insight; the research programme that followed built on it rather than within it.

Key works

See also: Darwinism · Williams · Dawkins