The development of P. falciparum is heavily dependent on the temperature. Since it requires a minimum temperature of about 20°C for the completion of sporogony inside the mosquito, climatic conditions during the Neolithic period were in fact substantially more favourable for the spread of P. falciparum and its vector mosquitoes into southern Europe than they were in the first millennium
or any other period after the Neolithic. What are now the Saharan and the Arabian deserts also received substantially more rainfall during the mid-Holocene climatic optimum than they do today, creating more breeding sites for mosquitoes.¹⁰ This would have assisted the spread of malaria from tropical Africa towards the southern shores of the Mediterranean. It is even conceivable that the geographic range of members of the Anopheles gambiae complex, the most important vector of malaria in tropical Africa today, may have extended further north in Africa than it has done in recent times. Mosquitoes can evolve very rapidly. For example, populations of Culex pipiens confined to London Underground tunnels and separated from above-ground populations have evolved new host preferences (mice, rats, and humans instead of birds), reproductive isolation from above-ground populations, new mating patterns (stenogamy instead of eurygamy), loss of winter diapause, and the possibility of oviposition without prior ingestion of a blood meal, all in no more than about one hundred years. Similarly the most ⁸ Zulueta (1973) and (1987); Sallares (1995) on the mid-Holocene optimum in western Eurasia. Recent research in China, reported in Nature, 390 (1997: 209), confirms that it was a worldwide phenomenon. It is estimated that the mean annual temperature was 2–4˚C
warmer, with 20–40% more rainfall, in China in the period 6000–2000 .
⁹ Baroni and Orombelli (1996).
¹⁰ Claussen and Gayler (1997).
30
Evolution of malaria
recent research in molecular evolution suggests that speciation in the A. gambiae complex in Africa is an active, ongoing process.
Mario Coluzzi has cogently argued that the modern strains of A. gambiae, which are exceedingly efficient at transmitting malaria, are of recent (Neolithic period onwards) origin.¹¹ The evolution of these extremely efficient vector strains would have permitted an increase in the transmission rate of P. falciparum malaria in tropical Africa. Coluzzi has suggested that an increase in the pathogenicity of P. falciparum might have accompanied the increased transmission rate. Recent research in molecular diversity indicates that the populations of the A. gambiae complex have been increasing recently but have also long had a large effective population size (the size of the breeding population). This suggests that its potential as a vector of malaria in Africa does go back to prehistory and is not a product of human population growth in modern times in Africa, even if it was not quite as efficient a vector then as it is today.¹² One population of Pygmies (hunter-gatherers until very recently) in central Africa has a very high frequency of haemoglobin S, a mutation which confers resistance to P. falciparum malaria on heterozygotes for the sickle-cell trait. Cavalli-Sforza argued that this implies that P. falciparum malaria was already widespread in central Africa before the invention or spread of agriculture in Africa.¹³ Because of the heat, the Neolithic period was the most likely period for the spread of P. falciparum malaria into southern Europe. Its previous evolutionary history, as has just been argued, suggests that the small human population sizes of that period would not have prevented it from becoming endemic in southern Europe then. Malaria is a disease that tends to exist in small foci because mosquitoes generally do not fly far from their breeding grounds, not more than five or six kilometres under Mediterranean environmental conditions, although occasional much longer migrations have been recorded.
In 1959 a migration of about 280 kilometres by the Egyptian mosquito species Anopheles pharoensis reintroduced malaria to Gaza and the coast of Israel, from which it had previously been eradicated.
¹¹ Byrne and Nichols (1999) on the London mosquitoes; Coluzzi et al. (1979); Coluzzi (1999).
¹² Lehmann et al. (1998); Powell et al. (1999).
¹³ Cavalli-Sforza (1986: 153–5, 416–19). Pygmy populations in forest environments, which A. gambiae is reluctant to enter, do not have high frequencies of haemoglobin S. Phylogenies based on mitochondrial DNA sequences suggest that some of the Pygmies are among the most ancient human populations.
Evolution of malaria
31
A propensity towards such migrations by this important malaria vector-species in the lower Nile valley might have helped P. falciparum to break out of Africa in the distant past.¹⁴
J. L. Angel used to invoke the high frequency of porotic hyperostosis in crania belonging to skeletons recovered from Mesolithic–Neolithic archaeological sites in Greece as evidence for a high frequency of P. falciparum malaria in Greece at that time.
There is now a general consensus among those interested in this problem that porotic hyperostosis, whose proximate cause is an iron-deficiency anaemia, has several other possible ultimate causes besides malaria. Consequently porotic hyperostosis cannot be used as evidence for the existence or frequency of P. falciparum malaria in Europe in the Neolithic period.¹⁵ However, absence of evidence is not equivalent to evidence of absence. It still remains quite possible that it was spreading in that period. Recently the application of the techniques of molecular biology to ancient biomolecules has opened up new avenues of research. Immunological tests have been used by two different research groups to identify the histidine-rich protein-2 antigen of P. falciparum in the mummies of several predynastic individuals from Egypt, dating to c.3200
