Possible Common Mechanisms of Macroevolution and Microevolution: A Possible Basis of Episodic, Sudden, and Relatively Constant Changes in the Fossil Record
Copyright ã 2001 by James Michael Howard. (Last sentence of paragraph 1 added March 22, 2010.)
It is my hypothesis that eukaryotic organisms evolved because of positive effects of dehydroepiandrosterone (DHEA) on replication of nuclear and mitochondrial DNA. Prior to this, I developed my hypotheses that the nuclear membrane and histones evolved to control separation of replication of nuclear and mitochondrial DNA necessary for cell duplication (1979). I suggest DHEA exerted profound effects on gene expression via their interaction with histones, nonhistone chromosomal proteins, and the free carbonyls available with the minor groove of DNA, that is, gene expression has been directly affected by DHEA.
I subsequently developed hypotheses suggesting that further evolution was stimulated by environmental (and endogenous) forces which accentuated the positive effects of hormones on replication and transcription of DNA. Therefore, I concluded that DHEA, and other hormones which affected DHEA production, were involved in the evolution of mammals. That is, animals which evolved into mammals were able to survive either/or the prolonged cold and dark of a meteorite impact or prolonged volcanic activity which removed the dinosaurs. DHEA increases heat, therefore, increases in DHEA eventually resulted in warm-blooded animals (mammals) and their other characteristics (Hormones in Mammalian Evolution, Rivista di Biologia / Biology Forum 2001; 94: 177-184).
DHEA production continued to have an impact on mammalian evolution because of subsequent, periodic increases in cold. Additionally, advantages in reproduction as a result of increased testosterone occurred which, I think, eventually produced primates and hominids. Testosterone production eventually became the driving force of human evolution. (Androgens in Human Evolution, Rivista di Biologia / Biology Forum 2001; 94: 345-362).
My understanding of the effects of testosterone on human evolution eventually produced hypotheses regarding, what I consider obvious, effects of testosterone on modern civilizations. That is, I think the effects of increasing testosterone are identifiable within living human populations. When testosterone begins to reach excessive levels, the phenomenon known as the "secular trend" occurs. (The secular trend is the increase in weight and height and earlier puberty often identified in children.) I also identified negative effects of excessive testosterone within populations. Two of the most negative phenomena include increased vulnerability to infections and reduced sperm count. A global decline in spermatogenesis and a global increase in infections are currently occurring.
It became clear to me that these two negative effects of testosterone could account for "Mitochondrial Eve" and "Y Chromosome Adam." ("Mitochondrial Eve," "Y Chromosome Adam," Testosterone, and Human Evolution, Rivista di Biologia / Biology Forum 2002; 95: 319-326). That is, I think excessive testosterone may have occurred within hominid populations. This began with the increase in female-to-male size in Homo erectus. That is, the increase in the female-to-male size observed in H. erectus indicated that the females increased in testosterone. As testosterone increased to excessive amounts, I suggest infections and reduced spermatogenesis reduced the hominid population allowing survival of small numbers of females. This is "Mitocondrial Eve." "Y Chromosome Adam" represents the event which increased spermatogenesis within the group of declining hominids. This was due to the transfer of the gene, DAZ (Deleted in Azoospermia), onto the Y chromosome. Spermatogenesis increased the population beyond the loss due to infections. This allowed testosterone to increase further and continue evolution of hominids.
It occurred to me that hormones are probably involved in the "Cambrian Explosion." Investigators now think a major meteoric event occurred prior to this event. This would have produced the same geological conditions that occurred because of the probable meteorite impact which removed the dinosaurs and paved the way for mammals. Organisms which lived through this major event would have significant advantages. They would have an increased ability to alter their DNA through crossing over and there would be far less competition for resources in the post-meteoritic world. There would be an "explosion" of new organisms and their ability to survive. Michael E. Baker suggests: "The adrenal and sex steroids receptor clade arose from an ancestral nuclear receptor in a primitive vertebrate at least 540 million years ago during the early Cambrian." (Mol. Cell. Endocrinol. 2001; 175: 1-4).
I think hormones are directly involved in geologically-associated, sudden, substantial changes in organisms and gradual, minute changes in organisms. Hormones may participate in "macroevolution" and "microevolution."