Professor Leslie Orgel, a British-born theoretical chemist and biochemist, was best known for his studies on how primitive life began on Earth roughly 4.5 billion years ago. His research was focused on discovering the nature of the chemical reactions that might have led to the origins of life.

He searched in particular for the precursor of ribonucleic acid (RNA), the molecule that handles much of the information processing of cells and is believed to have been the first molecule to replicate itself and be involved in the synthesis of proteins, which are essential to life.

RNA is crucially important in the processes by which genetic information is transmitted from deoxyribonucleic acid (DNA) into proteins. RNA is very similar to DNA, but differs in some important structural details.

Orgel was, in the 1960s, one of the first scientists to suggest that RNA, rather than DNA, was the first replicative molecule, passing on the genetic blueprint to future generations. He argued that DNA is too complex to be the first repository of genetic information, and that RNA probably preceded it. But, because RNA is also a very complicated substance, he synthesised simpler molecules that might have preceded it. RNA might, therefore, have evolved in a way consistent with Darwin's theory of selection. The theory that life is RNA-based life was shared by the late Francis Crick (obituary, July 29, 2004), the scientist who helped to discover the structure of DNA.

During his research, Orgel and his colleagues discovered an economical way to make cytosine arabinoside, commonly called cytarabine, used mainly in chemotherapy, particularly in the treatment of leukaemias and lymphomas. Cytarabine is also an antiviral agent that has been used for the treatment of herpes infection.

Leslie Eleazer Orgel was born in 1927 in London. He became interested in chemistry as a teenager, when he spent much time making and detonating explosives. He earned his bachelors degree in chemistry with first-class honours from Oxford in 1949. In 1950 he was elected a Fellow of Magdalen College, Oxford, and in 1951 received his PhD in chemistry. He continued doing research in chemistry at the university.

In 1954 he accepted a research fellowship at the California Institute of Technology, Pasadena, and in 1955 one at the University of Chicago. In 1955 he returned to Britain and became the assistant director of research in the University of Cambridge theoretical chemistry department, where he helped to develop the ligand field theory that describes chemical bonding in metals.

In 1964 Orgel was appointed senior Fellow and research professor at the Salk Institute for Biological Studies, La Jolla, California, where he directed the chemical evolution laboratory. He was also a professor in the department of chemistry and biochemistry at the University of California, San Diego. He became an American citizen in the 1980s.

He was one of five principal investigators in the research and training programme in exobiology (the search for the origin of life on the Earth and in the Universe) sponsored by Nasa. He participated, in 1975, in Nasa's Mars Lander programme (using spacecraft to conduct prolonged scientific studies on the surface of Mars), as a member of the molecular analysis team that designed the gas chromatography mass spectrometer instrument that robots took to Mars.

In 1998 he was the chairman of the US National Research Council task group on sample return from small solar systems bodies, and served on Nasa's astrobiology oversight committee. He was also a member of a strategic planning group for the SETI Institute's Centre, Mountain View, California, for the study of life in the Universe.

Although he was officially retired, Orgel continued his research until just before he died. He believed that evolution was “one of the interesting mysteries of science”. But, in thinking about how a universal genetic code could have evolved, he proposed that interstellar micro-organisms, such as spores and bacteria, may have been sown by a higher intelligence, coming to Earth from outer space, perhaps on meteorites, an idea supported by Crick.

In an article in the journal Icarus, they admitted that the scientific evidence for his proposition was so inadequate that it was impossible to say anything about its probability. Writing in Nature in 2006, Orgel stated that, apart from the rudimentary estimate that life began on Earth some 4.5 billion years ago, almost everything else about the origin of life remains obscure.

Orgel was also known for his “second rule”, which states that “evolution is cleverer than you are”, meaning that the trial-and-error methods of evolution can produce better results than centralised human planning, an argument often used to counter creationist arguments.

In addition to more than 300 scientific papers, he wrote An Introduction to Transition-Metal Chemistry: Ligand Field Theory (1960); The Origins of Life: Molecules and Natural Selection (1970); and he co-wrote, with Stanley L. Miller, The Origins of Life on the Earth (1974).

In 1957 Orgel was awarded the Harrison Prize for his work in inorganic chemistry. In 1962 he was elected a Fellow of the Royal Society. In the US he received a Guggenheim Fellowship in 1971, the Evans Award from Ohio State University in 1975, and the H.C. Urey Medal from the International Society for the Study of the Origin of Life in 1993. He was elected a member of the American National Academy of Sciences and of the American Academy of Arts and Sciences.

Orgel was known to his colleagues and friends as intellectually demanding and vigorous with great clarity of thought, but also a very generous and courteous man.

His wife, a daughter and two sons survive him.

Professor Leslie Orgel, chemist and biochemist, was born in on January 12, 1927. He died of pancreatic cancer on October 27, 2007, aged 80