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Science 14 August 1970:
Vol. 169. no. 3946, pp. 641 - 646
DOI: 10.1126/science.169.3946.641
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Articles

A Multiple Origin for Plastids and Mitochondria

Many independent symbiotic events may have been involved in the origin of these cellular organelles

Peter H. Raven 1

1 Department of Biological Sciences, Stanford University, Stanford, California 94305

The impressive homologies between mitochondria and plastids, on the one hand, and procaryotic organisms, on the other, make it almost certain that these important cellular organelles had their origin as independent organisms. The vast number of symbiotic relationships of all degrees of evolutionary antiquity which have been found in contemporary organisms point to the ease with which such relationships can be established.

The presence of drainage agriculture probably indicates that populations exceeded the carrying capacity of the more easily cultivated land. When populations declined, following the conquest by Europeans, the drained fields were probably given up because of the large amount of labor required to cultivate them. Most of these lands are now used only for cattle, but could undoubtedly be reclaimed again for agriculture (46). However, the cost of such reclamation might be prohibitive where populations are sparse, transportation is poor, and distances to markets are long. The chinampas of Mexico are an exception, since there is a major urban market close at hand.

The aboriginal cultures responsible for drainage ranged from the seminomadic Guato, to farm village and chiefdoms, to the high civilizations. The ridged-field farmers of the savannas were capable people but certainly not comparable to the farmers of the efficient and sophisticated states of the Andes and Mexico. Most of the drained-field systems could have been construted and managed through small-scale family and community co-operation, as in New Guinea, rather than through central political control. On the other hand, there does seem to be a frequent relationship between degree of agricultural intensification, population size, and complexity of social organization (47). Unfortunately, not much is known yet about the intensification process for drained-field cultivation or for other reclamation systems.


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