AL-BIRUNI (973-1050)

The Persian scholar al-Biruni lived around the same time as ibn-Sina. He pioneered the idea that light travels faster than sound, promoted the idea that the Earth rotates on its axis and measured the density of 18 precious stones and metals.

portrait of al-biruni

He classified gems according to the properties: colour; powder colour; dispersion (whether white light splits up into the colours of the rainbow when it goes through the gem); hardness; crystal shape; density.
He used crystal shape to help him decide whether a gemstone was quartz or diamond.

He noted that flowers have 3,4,5 or 8 petals, but never 7 or 9.

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CHARLES LYELL (1787-1875)

1850 – UK

‘An Attempt to explain the Former Changes of the Earth’s Surface by Reference to Causes Now in Operation’


At the start of the nineteenth century, most people believed that a few major events had shaped the Earth, one of which was Noah’s great biblical flood. In between these catastrophic events the Earth had remained unchanged.

Charles Lyell replaced catastrophe theory with uniformitarianism, which proposed that the Earth changed gradually as constantly present forces acted upon it. He attributed ages to rock strata, by looking at the fossils they contained. This introduced a way of studying the Earth and led to modern geology. Lyell started a chain of thought that has now generated a complex understanding of the Earth’s history, allowing it to be divided into discrete eons, eras, periods and epochs.

There is evidence that Darwin was influenced by Lyell, although Lyell was deeply troubled by Darwin’s concept of natural selection. Darwin wrote “The greatest merit of the Principles (of Geology) was that it altered the whole tone of one’s mind, and therefore that, when seeing a thing never seen by Lyell, one yet saw it through his eyes.”

Lyell believed that geological, and therefore biological, history was cyclical. While Lyell destroyed one major dogma, his adherence to other ideas prevented geology moving forward.

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Geological evolution – Charles Lyell


1859 – England

‘All present day species have evolved from simpler forms of life through a process of natural selection’

Portrait of Charles Darwin ©

Organisms have changed over time and the ones living today are different from the ones that lived in the past. Furthermore, many organisms that once lived are now extinct.

The orthodox view was that of the Creationists. According to the Book of Genesis in the Bible, ‘God created every living creature that moves….’. Against this background, thinkers such as French naturalist Jean-Baptist Lamarck developed a picture of how species evolved from single-celled organisms.

Darwin’s breakthrough was to work out what evolution is and how it happens. His insight was to focus on individuals, not species and to show how individuals evolve by natural selection. The mechanism explained how all species evolved to become well suited to their environment. Later commentators have characterized this idea as ‘survival of the fittest,’ but this was never a phrase that Darwin himself used.

Darwin was influenced by CHARLES LYELL’s newly published book ‘Principles of Geology’, showing how landscapes had evolved gradually through long cycles of erosion and upheaval and by ‘An Essay on the Principle of Population’ written in 1798 by THOMAS MALTHUS.

The publication of Darwin’s book ‘On the Origin of Species by Means of Natural Selection’ in 1859 generated social and political debate that continues to this day. Darwin did not discuss the evolution of humans in this book.
In ‘The Descent of Man’, published in 1871, he presented his explanation of how his theory of evolution applied to the idea that humans evolved from apes. In modern form the theory contains the following ideas:

  • members of a species vary in form and behaviour and some of this variation has an inherited basis

  • every species produces far more offspring than the environment can support

  • some individuals are better adapted for survival in a given environment than others

this means that there are variations within each population gene pool and individuals with most favourable variations stand a better chance of survival – the survival of the fittest.

  • the favourable characteristics show up among more individuals of the next generation

there is thus a ‘natural selection’ for those individuals whose variations make them better adapted for survival and reproduction.

  • the natural selection of strains of organisms favours the evolution of new species, through better adaptation to their environment, as a consequence of genetic change or mutation.

Knowledge of DNA has enriched the theory of evolution. The modern view is still based on the Darwinian foundation; evolution through natural selection is opportunistic and it takes place steadily.

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1915 – Germany

‘Continental land masses are constantly in motion. The Earth’s land surface was once one big super-continent. About 250 million years ago it broke up into the continents we know today, which have since drifted to their present positions’

Photograph of ALFRED WEGENER ©


Wegener proposed that the continental land masses are moving over the face of the Earth.
Rock under the ocean is principally Basalt, a denser rock than the Granite that makes up the continents. At the start of the Earth’s history there was just a single landmass, which began to break up 200 million years ago, and the parts are still moving. Mountain ranges have been produced where one moving land mass crashes into another, pushing rocks together and forcing them upwards in folds. The tectonic plates move over the asthenosphere carried by convection currents in the magma below.

Up until and beyond Wegener’s death his ideas had little scientific credence – until in the 1950s the mid-Atlantic ridge was discovered. It was this discovery that led to the concept of the tectonic plates.




1935 – USA

‘A scale ranging from 0 to 9 to measure the magnitude of earthquakes’

photo of CHARLES RICHTER who devised a scale for measuring the magnitude of earthquakes ©


The Richter scale is a numerical scale that gives the magnitude of an earthquake by calculating the energy of shock waves at a standard distance. The scale is logarithmic, so each additional point represents a tenfold increase in severity. Thus a magnitude 7.0 earthquake is 10 times as powerful as one of magnitude 6.0 and 100 times as powerful as one of magnitude 5.0.
In terms of energy, one unit represents an increase in the energy of roughly 3 times. A magnitude 7.0 earthquake unleashes about 1000 times the energy released by a magnitude 5.0 earthquake.

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WILLARD LIBBY (1908- 80)

1946 – USA

‘Radiocarbon can be used to estimate the age of any organic material. The radioactive isotope of carbon,14C (carbon-14) is present in all living things. When life stops 14C begins to decay. From the rate of decay the age (or time of death) of an organism can be calculated’

The two most common forms of carbon 12C and 13C, make up virtually all types of carbon and are stable – 12C is the simplest form and is made up of 6 protons and 6 neutrons; 13C is slightly heavier because it has one more neutron. 14C, known as radiocarbon has the unstable combination of 6 protons (defining it as carbon) and 8 neutrons.

In the late 1940s Libby led the team at the University of Chicago, USA, that developed radiocarbon dating using the radioactive isotope 14C.

Living things go on absorbing 14C until the time of their death. The half-life of 14C is 5730 years – once an organism dies, 14C begins to decay. As a result the ratio of 12C to 14C changes with time. By measuring this ratio, it can be determined when the organism died.

Libby suggested that minute amounts of radiocarbon come from the upper part of the atmosphere. He put forward the idea that when high-energy particles formed in deep space – cosmic rays – reach the atmosphere, they interact with nitrogen gas to form radiocarbon. He argued that the newly formed radiocarbon is rapidly converted to carbon-dioxide, CO2, and is taken up by plants during photosynthesis; with the result that the radiocarbon enters the food chain. Everything alive should therefore have the same radiocarbon concentration as the atmosphere.

Once an individual dies, some of the 14C atoms begin to disintegrate and give off an electron to reform nitrogen. Libby argued that if the original radiocarbon content is known. it should be possible to measure the remaining 14C in a sample of tissue to back-calculate its age, in a similar way to estimating how much time has passed by measuring the amount of sand left in the top of an egg timer.
By the end of the 1940s, Libby and his team had shown that the radiocarbon content of the air was the same around the world and that 14C could be used to date anything organic.

The crucial principle is the half-life of the unstable atom, the rate at which it will break down. The longer the half-life of a material, the further back in time a dating method can go. With radiocarbon, the dating range is 40,000 to 60,000 years.

When Libby originally measured the half-life of radiocarbon, he calculated it to be just over 5720 years. During the 1950s a new estimate of 5568 years was made by other researchers, who assumed that Libby had got his figures wrong and the 5568-year half-life was adopted by the scientific community.
It is now known that the half-life of radiocarbon is 5730 years, virtually identical to Libby’s original estimate. As a result of the large number of samples that had already been dated, the incorrect value of 5568-years is used in estimates – confusingly this is now termed the ‘Libby half-life’. As all labs use the same half-life value, all ages are directly comparable.

With radiocarbon dating the assumptions made are:

  1. that the atmosphere has had the same 14C content in the past as today
  2. that all things alive have the same radiocarbon content as one-another and as the atmosphere
  3. that no more radiocarbon is added to a sample after death

To obtain a final radiocarbon age, we have to use a point in time to compare against. 1950 is used as year zero and all ages are described relative to this as ‘before present’ (BP). Radiocarbon dating does not give a precise date and estimates are given within a range of uncertainty.

Libby received numerous awards for this work,including the 1960 Nobel Prize for Chemistry. Libby also worked on the Manhattan Project during World War II, helping to enrich the uranium used in the atomic bombs.

picture of the Nobel medal - link to

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