(Selected Tables and Figures referenced, but not present in this blog
can be found in their corresponding Science Version blogs)
Strictly speaking, the evolutionary development of the immune system should be the first topic of discussion, but I postponed it to after Blog #5 so as to provide a review of the biology that is a significant part of the immune system’s evolution. I actually scattered evolution discussions throughout the book rather than a dedicated chapter and in retrospect, after doing some post-publication research on immune evolution and realizing its importance, I could have kicked myself. I assure you it will warrant a full chapter in the next edition of the book, but in the meantime, the next two blogs will cover some of the most valuable information on evolution and the immune system.
Around 600 million years ago, small living organisms began to evolve, and then over the following few million years, they began to develop sophisticated biological systems including an immune system. They call this period the “immunologic big bang.” Using Darwin’s natural selection, it included the evolution of the blood cells that control the immune response (remember lymphocytes?) and the surface proteins those cells use for identifying antigens, and the cells' genetic processes that created DNA and RNA. It was during this “big bang” period that the innate immune system formed to provide our self-defense against foreign microbes, called pathogens and it produced a protective mechanism against them called inflammation. Higher forms of developing animals also began to develop a more aggressive “adaptive immune system” as further protection against those deadly pathogens.
Besides Darwin’s natural selection, another type of evolution was developing called the Lamarckian “Theory of Inheritance of Acquired Characteristics.” This theory explains the ability of a developing organism, rather than using natural selection, to pass on physical characteristics to its offspring that had been originally developed through use or disuse over time. This theory continues to be confirmed, even today with development of a new generation of gene editing tools, e.g., CRISPR that we'll cover in great detail in future blogs.
Most healthy humans have the ability to activate and utilize the multitude of immune responses that they are born with. But individuals differ in that degree at birth, during aging, sex differentiations, and when encountering new environmental factors. The ability to utilize the range of immune responses is rooted in our evolutionary pasts where genes develop to control the vast array of immune traits that collectively make up the human genome. These evolutionary roots apply to genetic differences between males and females and are directly related to the risks for immune-related diseases including autoimmunity and infection.
Viruses, bacteria, fungi and other microbes existed in the cosmic environment prior to human evolution more than 3 billion years ago. So, the human genome, the host for all these colonizing microbes, eventually evolved the human microbiome, also called "the second genome" (Blog #13). Through natural selection, the microbiome, along with the coexisting environmental microbes create a harmonious balance beneficial to the host, i.e., the human, and other species living in or around it. More on this phenomenon in the embryology and pregnancy blog #8. But first, let’s continue with Part 2 of evolutionary development in Blog #7.
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