Convergent evolution and disagreement about morphology have left speculation about nematode origins wide open. Each square meter of garden soil contains up to 20 million nematodes, most of them smaller than 1mm in length. Nematodes are particularly populous in sandy tropical and desert valley soils. Heavier clay soils are less aerated and support fewer organisms.
Because nematodes are small and soft, the fossil record for their evolution is limited. In a 1994 article, George O. Poinar Jr. of the University of California at Berkeley and two other researchers reported that the oldest known nematode fossil had been found in Lebanon. A nematode of the Mermithidae family was coiled inside the gut of a biting midge that became entrapped in sticky plant resin (amber) during the Cretaceous period, 130 million years ago. The midge itself was a blood parasite, and the Mermithidae was a parasite inside a parasite.
For some genera, nematode history is reconstructed by comparing distribution to continental drift. Genus Xiphenema, for example, is found worldwide, but it is most diverse in Africa. A closely related genus, Xiphidorus, is South American. In the Triassic period, 300 million years ago, all continents were joined as Pangaea. Then, 200 million years ago, Pangaea broke into northern Laurasia and southern Gondwana. During the Cretaceous, Gondwana further separated into Africa and South America. To achieve a worldwide distribution but still have a close relative in South America, researchers determined, Xiphenema must have spread out of Africa before Pangaea broke up but not before Gondwana separated.
Free-Living to Parasite
The history of nematodes shows an evolution from free-living organisms in water and soil to highly specialized, dependent parasites. Free-living marine nematodes are often the most abundant organism in estuary and marine sediments, where the extent of their biodiversity can be used to measure the ecosystem's health. Fifty percent of known living nematode species are plant or animal parasites. For some nematodes, history can be reconstructed based on the fossils of plants or animals that are exclusive hosts of those nematodes.
Human Vs. Nematode Part 1
Traditional methods of controlling nematodes in garden soil relied on crop rotation and allowing fields to sit fallow so that root parasites had no hosts. The soil fumigant carbon disulfide was first used in French vineyards in the 1880s. Surplus World War I teargas was used for soil fumigation until supplies were exhausted. In the 1940s, various pre-planting chemical fumigants were developed. By the 1960s, organophosphates and carbamates were successfully applied on fields among living plants.
Human Vs. Nematode Part 2
The National Sustainable Agriculture Information Service, which is funded through the U.S. Department of Agriculture, recommends organic nematode control. Rotating unrelated crops prevents nematodes from attacking the same host each year. Adding composted organic matter improves soil structure and releases plant toxins lethal to nematodes, and it also increases fungus and bacteria that parasitize nematodes. Minimum tillage maintains a high number of nematoparasites. Nemato-suppressive soil amendments include oil-cakes, sawdust, sugarcane bagasse, bone meal, horn meal and green manures such as cereal rye. Chitinous amendments such as crushed shells of shrimp and crab nourish fungi that also eat chitin in nematode eggs.