"Mitochondrial Eve," "Y Chromosome Adam," Testosterone, and Human Evolution
Rivista di Biologia / Biology Forum 2002; 95: 319-326
Copyright ã 2002 by James Michael Howard.
I suggest primate evolution began as a consequence of increased testosterone in males which increased aggression and sexuality, therefore, reproduction and success. With time, negative effects of excessive testosterone reduced spermatogenesis and started a decline of the group. Approximately 30-40 million years ago, the gene, DAZ (Deleted in AZoospermia) appeared on the Y chromosome, increased spermatogenesis, and rescued the early primates from extinction. (Note: DAZ is considered by some to specifically, positively affect spermatogenesis; others suggest it has no effect on spermatogenesis.) Hominid evolution continued with increasing testosterone. The advent of increased testosterone in females of Homo erectus (or Homo ergaster), increased the female-to-male body size ratio, and eventually produced another era of excessive testosterone. Excessive testosterone caused a reduction in population size (bottleneck) that produced the "Mitochondrial Eve" (ME) mechanism. (Only certain females continued during the bottleneck to transmit their mitochondrial DNA.) That is, the ME mechanism culminated, again, in excessive testosterone and reduced spermatogenesis in the hominid line. Approximately 50,000 to 200,000 years ago, a "doubling" of the DAZ gene occurred on the Y chromosome in hominid males which rescued the hominid line with increased spermatogenesis in certain males. This produced the "Y Chromosome Adam" event. The doubling of DAZ allowed further increases in testosterone in hominids that resulted in the increased size and development of the brain. Modern humans periodically fluctuate between the positive and negative consequences of increased levels of testosterone, currently identifiable as the secular trend, increased infections, and reduced spermatogenesis.
It is my hypothesis that human evolution is primarily a consequence of the effects of androgens on gene regulation within a relatively stable genome over time (Howard [2001a]). That is, our evolution is an extension of ongoing mammalian evolution (Howard [2001b]), primarily accelerated by testosterone. Increasing testosterone probably participated in the formation of primates, and later, the formation of hominids. The mechanisms, designated "Mitochondrial Eve" and "Y Chromosome Adam," represent two aspects of changes that occurred because of effects of testosterone on hominid reproduction.
"Mitochondrial Eve" resulted from an advantageous increase in testosterone production by female hominids that positively affected reproduction but subsequently resulted in decline of the population. This event produced larger females which increased the female-to-male ratio in hominids and produced the large hominid, Homo erectus (or H. ergaster). This combination of males and females of high testosterone produced the increase in body and brain size of H. erectus. I suggest excessive testosterone may produce negative effects which eventually, adversely affected reproduction of H. erectus. As the population declined, only certain females were able to continue reproduction. Therefore, only the mitochondria of this "set" of female hominids would be continued in the population.
The increase in testosterone of the Mitochondrial Eve (ME) mechanism produced increased aggressiveness, advantageous to reproduction because of increased sexuality and dominance in males and females. This is why H. erectus was so successful. ME and her offspring were sexier and drove others away, thereby, concentrating her genes. ME is still with us; the mechanism has not changed. In modern humans, it produces the "secular trend," the increase in size and height and earlier puberty currently occurring in children in the U.S.A. (Freedman et al. ). I suggest the secular trend is actually an increase in the percentage of people who produce increased testosterone. Therefore, they are more aggressive and sexual; they reproduce faster than those of lesser testosterone.
I think Homo erectus developed because of increased sexuality in both males and females. It is my suggestion that populations that include females that exhibit increased sexuality also have an advantage in reproduction in free-living primates. This exact situation has been identified in free-living Bonobos.
"Differences in social relationships among community members are often explained by differences in genetic relationships. The current techniques of DNA analysis allow explicit testing of such a hypothesis. Here, we have analyzed the genetic relationships for a community of wild Bonobos (Pan paniscus) using nuclear and mitochondrial DNA markers extracted from faecal samples. Bonobos show an opportunistic and promiscuous mating behaviour, even with mates from outside the community. Nonetheless, we find that most infants were sired by resident males and that two dominant males together attained the highest paternity success. Intriguingly, the latter males are the sons of high-ranking females, suggesting an important influence of mothers on the paternity success of their sons." (Gerloff et al )
Y Chromosome Adam
The ME mechanism should occur most rapidly in the most propitious circumstances, that is, favorable "feed and breed" areas. This would keep the group together sufficiently long to produce the ME mechanism and increase testosterone. Increased aggression occurs coincidentally with increased sexuality, therefore, high testosterone males and females should concentrate within an area, with lesser testosterone types at the periphery and beyond. This is very beneficial for this type of breeding colony. However, this situation ultimately produces problems for hominids. Excessive testosterone will ultimately produce consequences which reduce fertility. The population can expand only so much before negative effects of testosterone begin to accrue.
Among a number of negative effects of excessive testosterone, reduced immune response and reduced fertility produce the most adverse effects. In mammalian animal models, testosterone reduces resistance to viruses (Holyoak et al. ; McCollum et al. ), bacteria (Yamamoto et al. ), and lowers immune function following soft-tissue trauma and hemorrhagic shock (Wichmann et al. ). Too much testosterone increases the probability of infection. This would be especially detrimental to a group of hominids engaged in periodic aggression.
Too much testosterone may reduce spermatogenesis. Testosterone is currently being considered as a male contraceptive. Increasing testosterone adversely affects spermatogenesis in men (Zhengwei et al. ; Ge et al. ). I suggest natural, gradual increases in testosterone within a population may gradually reduce spermatogenesis sufficiently to reduce overall fertility in a hominid population.
Therefore, increases in the percentage of individuals of higher testosterone may produce declines in a population because of increasing infections in males and females and reduced spermatogenesis in males. Young, aggressive males might not live to reproduce coincidentally with declines in dominant male reproduction. This situation could significantly reduce the life span of a population, perhaps to extinction.
The gene DAZ (Deleted in AZoospermia) is necessary/beneficial for spermatogenesis (Thielemans et al. ). Since increasing testosterone may have participated in the formation of original primates, excessive testosterone may have participated in their potential decline. DAZ first arose approximately 30-40 million years ago at the approximate time of the beginning of primates (Xu et al ). DAZ may have improved spermatogenesis enough that reproduction increased sufficiently to overcome the negative effects of testosterone in early primates.
The same mechanism that started, then reduced, then rescued the early primates may have repeated itself in the evolution of hominids. Approximately 143,000 years ago, these groups may have been reaching their viable limits due to excessive testosterone. I suggest "a" group, or a very limited number of very similar, related groups (Mitochondrial Eve), was again altered by DAZ mutations which affected spermatogenesis sufficiently to continue hominid evolution. This is supported. Approximately 55,000 to 200,000 years ago, DAZ doubled on the Y chromosome (Agulnik et al. ). The "Y Chromosome Adam" is dated approximately to 59,000 years ago. This doubling of DAZ may have increased spermatogenesis a second time and allowed hominid survival.
Homo erectus migrated to various parts of the world but did not survive. Homo neandertalensis did not survive. Both groups survived for lengthy periods in their environments. The doubling of the DAZ is thought to have occurred only in Africa (Agulnik ibid.). Only hominid groups in which the doubling of DAZ occurred may have survived the negative effects of too much testosterone.
"Mitochondrial Eve" and "Y Chromosome Adam" may represent mechanisms directly tied to levels of testosterone in hominids. I suggest ME was the result of the increase of testosterone in female hominids which increased the ratio of female-to-male size in Homo erectus. This increase in testosterone became excessive and caused negative effects on the fertility of the group, especially male fertility. The remaining females of this group are the source of surviving mitochondria. DAZ is tied to male fertility; some suggest DAZ positively affects spermatogenesis. The mutation of DAZ, which doubled the gene on the Y chromosome, may have rescued the surviving hominids from decreased fertility. Hominids carrying the double DAZ increased brain size as a result of the effects of their increased testosterone on brain growth and development.
It is my hypothesis that human evolution is driven by increases in testosterone. Since the mechanism is simply based on the increase in individuals of higher testosterone, I suggest it occurs today. The secular trend is its current signal; it is real and vigorous in the U.S.A. (see Freedman). We are also seeing an increase in infections and a decline in spermatogenesis in the U.S.A.
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