HUMAN EVOLUTION
M. Hckford Institut de Paldontologie 8, rue Buffon 75005 Paris, France
Key words: ~Unique mother
hypotheses~, Mitochondrial DNA, Human evolution, bottlenecks, biological variability, polymorphisms.
Vol. 6 - N. 3 (263-268) - 1991
Paradise lost: Mitochondrial Eve Refuted Being based solely on neontological data, all ~unique parent)~ evolutionary hypotheses, of which ~Mitochondrial Evo) is one, fall into the category of scala naturae. Mathematical treatment of neontological data bases, using cladistic approaches does n o t confer the status of scientific hypotheses onto such scenarios. Apart from these fundamental problems, such hypotheses are flawed on a number of other bases, including the fact that there is a proportion of parental contribution to mitochondrial lineages, despite widely publicised statements that mithocondrial DNA in mammals is ~strictly~) maternally inherited. Other weaknesses of <~unique mother~>hypotheses on that their proponents endeavour to describe the evolution of diploid organisms on the basis of variability in extant haploid organelles, the evolution of which is delinked from that of the diploid organism. A further difficulty is that it is not possible to reconstruct interspecific relationships on the basis of intraspecific variability. There is a general ignorance among proponents of <~unique mothen) hypotheses regarding the distribution of biological variability on the surface of the globe, a fact which renders the molecular clock inaccurate, and which upsets the simplistic proposal that molecular diversity equates with time. <~Uniquemother~) scenarios are also invalidated by the presence of shared chromosome and other polymorphisms in african great apes and humans at similar percentages in the different lineages, a fact which indicates that these evolving populations did not experience ~bottlenecks)~. These and other difficulties effectively refute the <~MitochondrialEvo) hypothesis, which in any case much resembles creationism of a special kind, in which the offspring of a breeding pair are visualised as belonging to a species different from its parents. Such extreme examples of the punctuational mode of evolution are highly likely to be incorrect.
<~Attempts to reconstruct the biological past solely on the basis of the living is dangerous: the road of anthropology concerning evolution is crowded with the debris of such efforts; several of them, following more recent discoveries of authentic human remains, or of better data, have been shown to be completely wrong~). (WILLIAM Howra.~s, 1986) ~dt is now clear that the molecular record rather than the fossil record is a more suitable indicator in clarifying the phylogenetic relationship among extant species and in dating the branching events among theme). (HAsEGAWA & YANO, 1984)
Mitochondrial Eve, a hypothesis of h u m a n origins based solely on a neontological data base, was first proposed b y BROWN (1980) and was more widely publicised b y CANNet al. (1987) and STONEKING & CANN (1989). I n brief, the hypothesis proposes that all 9 EditriceII Sedlcesimo- Firenze
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humans can trace their origins back to a unique <
>who lived in Africa about 200,000 years ago. For this reason, the hypothesis has been subjected to extensive media attention, where it is often known as the <>or <>hypothesis. It is the basis for other hypotheses including the <>or <>model of human origins. Recently, the <>of mitochondrial DNA, which was a cornerstone of the <>hypothesis, has been refuted, a proportion of mtDNA being of paternal origins (GYLLENSTENet al., 1991). These authors maintain that the <>hypothesis is still valid, but that the age of the last common ancestor should be revised upwards. In fact, the demonstration that mtDNA can contain molecules derived from the paternal line represents a major weakness of the hypothesis, already under attack for other reasons (DARLU& TASSY, 1987a, 1987b); EXCOFFIER& LANGANAY, 1989; EXCOFFIER 8~ ROESSLI, 1990; WOLPOFF, 1989). The <>hypothesis, like many other hypotheses based solely on neontological data bases, especially those analysed using mathematically rigorous cladistic methods, fails to describe biological history. The reasons for this are that neontological data bases are not endowed with the time dimension <>,even though they are the products of biological change which took place over time. The determination of the <>of characters used in the cladistic method is often compromised because cladists usually deny that time <>should play a role in determining phylogenetic relatioships between taxa. Time is the only known dimension which possesses polarity. Thus its denial by cladists engaged in phylogeny reconstruction is a contradiction. Furthermore, cladists usually try to depict in two dimensions the results of a five dimensional process: biological evolution. By adopting such a <>approach, all hypotheses based solely on neontological data bases, however analysed, are nothing more nor less than scala naturae. Rigorous mathematical treatments of such data bases in no way upgrade them to the status of scientific hypotheses. Apart from these general problems with neontological data bases, the < hypothesis suffers from other major difficulties, which have yet to be addressed by proponents of the hypothesis. The entire fabric of <> hypotheses (laboratory mice; FERRIS et aL, 1982: african apes, SPUHLER, 1988) is dependent upon the assumption that mitochondrial DNA (mtDNA) is of value in determining pathways and rates of evolution of the organisms in which they occur. Such may be the case for asexually reproducing organisms, but for sexually reproducing organisms, such as humans, this assumption is invalid. This is because mitochondria are haploid organelles, the evolution of which is delinked from that of their diploid host organisms. There are great difficulties in reconciling the evolution of haploid and diploid organisms, their replication/reproduction processes being so divergent. Indeed, it has been proposed that <>of haploid organisms cannot be defined in the same way that <>of diploid organisms are. Considering these difficulties in definition, it is doubtful that evolutionary events in a haploid organeUe, even if they could be determined, would map closely onto evolutionary events that occurred in the diploid organism as a whole. The reconstruction of evolutionary events in humans solely on the basis of mitochondrial molecules is thus fraught with uncertainties. Rather than representing a strength to the <> hypothesis, the haploid nature of mitochondria represents one of its major weaknesses. The same difficulties arise for all hypotheses based on haploid organelles, including those based on the haploid portions of the Y chromosome (LucoTTE, 1989). A second insurmountable difficulty with the <>hypothesis and others like it, is that it tries to model interspecific evolutionary history upon the basis of intraspecific
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variability. Even though the documentation of intraspecific variability is of interest, such data cannot on their own be used to reconstruct phylogenetic relationships between taxa at higher taxonomic levels, including those between species such as Homo sapiens and Homo erectus. The only valid approach for determining the relationships between these taxa, is to have data sets obtained from both species, which are then compared. A third flaw in the ~African Eve~ hypothesis is the assumption that variability within a species is solely a function of time (STvaNCER& ANDREWS, 1988). It has been known since the time of WALLACE (1876) that there are strong latitudinal components to biological variability, and from the time of HUMBOLDT(1805) that there is an analogous altitudinal component. Such latitudinal and altitudinal components of biological diversity are present in most organisms on earth, regardless of the ecosystem in which they occur (STEHLI, 1968). Latitudinal biological diversity gradients are usually presented as curves of species diversity or generic diversity within superfamilies or higher level categories, but the same principals apply at lower levels of taxonomic hierarchy. For example, Ibraimov, (in press) has demonstrated that individual variability in humans varies with latitude in much the same way that taxonomic variability does. Such variability is greatest in the lowland, humid tropics and reaches minimal values at high latitudes and altitudes. For this reason, the relatively great variability in the small sample of African mtDNA in comparison with that from humans that live at higher latitudes, cannot form the basis for proposing a greater age for african human populations, without first taking into account the effects on variability due to latitude and altitude. It should be noted that the main alternative contendor for the origin of humans, was the sample from are themselves the subject of debate. It should be pointed out that molecular biologists have been dreadfully inconsistent about their choice of palaeontological ~dix-points~, the various ones chosen being incompatible with each other to a marked degree. The only consistent part of the exercise seems to be that a ~fix-point~)is selected which will conform most closely with the desired running speed of the clock. The inherent probability of
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circular reasoning greatly weakens this line of argument, which represents an added difficulty for the <>hypothesis. < and similar hypotheses, being based on a cladistic approach to the analysis of data, usually arrive at the notion that species originate from <> or from populations that have gone through one or more <>. Indeed, <> is an extreme example of punctuated evolution, the daughter being a different species from the mother and father. The only more extreme form of evolution is <. This <> concept arises directly from cladistic analysis and is an artefact of the method of analysis. It has no biological foundation, and is easily refuted. For example, humans, chimpanzees and gorillas share a large number of chromosome polymorphisms (WmNBERG& STANYON, 1987), some of which occur in as few as 4% of the population. For such polymorphisms to be present in similar frequencies in three different genera of Hominoidea indicates that populations of the lineages leading to humans, chimpanzees and gorillas, must never have dropped below several thousand individuals. Either that, or there must have been an extraordinary degree of convergent evolution in chromosome polymorphisms. A further example of polymorphism helps to refute the <> concept. The frequencies of tasters to non-tasters of phenylthiocarbamide in humans, chimpanzees and gorillas is comparable in all three groups (roughly 25% are non-tasters). In the OrangUtan, the frequencies are completely different (roughly 90% are non-tasters). These data confirm that among the extant great apes and humans, none of the lineages went through population <, let alone evolved from <~. Under the circumstances, the <) hypothesis seems to be untenable, representing as it does an extreme example of the <> hypotheses. DARLU& TASSY (1987) have already dealt with the problem of lack of outgroup comparisons by which to <>the <~ is applicable to biological systems. CO~UCCINI (1991) has argued that in phylogeny reconstructions, the most parsimonious solution is almost certain to be incorrect. Biological systems are complex systems which reductionist approaches will always fail to describe accurately. This is because evolution is a five dimensional process, whereas cladistics is essentially a two dimensional analytic method. The second problem with the cladlstic approach, is that most of its proponents envisage most, if not all evolution as proceeding by cladogenesis (successions of dichotomies), whereas there are good reasons for considering that anagenesis (changes over time within lineages without dichotomies) is a common process of evolution (ring species and their chronological analogues). < represents an extreme example of punctuational evolution (mother and daughter belong to different species) followed rapidly by a dichotomy (african group versus the rest of the sample). As such it has elements of instantaneous <~ (speciation of a creationist kind without dichotomy) followed immediately by cladogenesis (dichotomy into two major groups). Both of these steps arise as artefacts from the method of analysis, and cannot be considered to describe the early stages of the evolution of Homo sapiens. A further argument against the <> hypotheses is of an ecological
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nature. It is clear from the fossil record that pre-sapient humans were widespread in the Old World. From this it is safe to say that H o m o erectus was eurytopic, being well adapted to a wide variety of habitats and living conditions, even if there were well developed cultural and micro-environmental reasons (use of fire, clothing and shelters) helping to maintain this eurytopy. A <> and her offspring would represent an extreme example of stenotopy, whereas H o m o sapiens is probably even more eurytopic than was Homo erectus. Even though details are not yet available, it seems likely t h a t sapient humans built upon the eurytopy already present in pre-sapient humans rather than redeveloping it afresh from scratch. We consider it to be more likely that for H o m o erectus to survive for nearly a million years in Africa, Europe and Asia, both as an evolving biological entity and as a cultural one, there must have been both gene flow and information flow throughout the species range. Such gene flow and information diffusion may not have been continuous everywhere through geological time, but, if there were periods of genetic and cultural isolation, then these were apparently never severe enough for full speciation of H o m o erectus into two or more species to occur even though the development of geographic > hypothesis and its logical offspring, the <> model of the origins of modern humans. Considering the above arguments, we conclude that all < hypotheses, of which <> is but one example, are refuted, as are all <,unique father~> hypotheses. They are untenable on many grounds, and merely add to the debris already cluttering the road of anthropology. The only useful purpose such hypothesis have served is to reveal the weakness behind all phylogenies based solely on neontologic data bases, and the inapplicability of two dimensional analyses to five dimensional systems. We conclude that those scientists who reconstruct phylogenies without taking into account the fossil record, do so at their peril. ACKNOWLEDGEMENTS- - I would like to thank Prof. A.B. Chiarelli for interesting discussions about the basis and implications of the <
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