Survival of the Fittest
A number of commentators have, over the last few years, expressed a belief that humans will most likely go extinct at their own hands. Or at very least they will revert to primitive behaviors (usually characterized as brutish) coming through a bottleneck by going back to caveman mentalities and actions. These sentiments are certainly understandable from the standpoint that this is the only experience our kind has known as we climbed up from savagery to become domesticated and “civilized.” But this thinking misses some important aspects of the evolution of more intelligent species. In the face of an almost certain collapse of civilization and the prospects of a bottleneck event we might easily think this will propel us backward. But this ignores what has actually happened in previous extinction/bottleneck events. I've been interested in the question of why birds and mammals did so much better during and after the extinction event, 65 million years ago, in terms of survival (as compared with the dinosaurs). Both groups enjoyed a rapid adaptive radiation of species subsequently to give rise to the age of mammals and birds as the dominant mega fauna. Something is going on here that is counter-intuitive. It is a key to claiming that a new bottleneck event for humans might not result in the outcomes so many have expressed as logical. I expect something more promising and I'll tell you why.
The dinosaurs, except for birds, a sub-clade of saurischian dinosaurs, did not survive on Earth. How is it that mammals and many birds did? Of course millions of species and thousands of genera lower in the phylogenetic tree survived as well even though millions more went extinct. The basic rule of evolution is that the fittest organisms (species) will differentially survive and procreate, thus out competing the less fit. But fitness is in reference to the conditions of the environment and econiche of the species. The Earth's hydro and atmospheres have always been in flux. The continents have always been shifting. The local ecologies have always been changing, sometimes rapidly, more often slowly. The more rapid changes are the ones that stress species and provide the major differentiating forces that do the selecting. And then some species are more able to operate under the changed conditions and compete against rivals for the niches available more successfully. Homo sapiens was just such a species, for example out competing the Neanderthals until as recently as 13,000 years before the present, when the latter appear to have died out.
The End-Cretaceous Event appears to have been a fairly abrupt one by geological and climatological standards. The prevailing current hypothesis holds that the event was triggered by an asteroid of sufficient mass slamming into the Yucatan around 65 mybp. Recent evidence adds strength to this hypothesis. The cataclysm had global effect, what we would call a nuclear winter-like phenomenon that so severely altered Earth's climate that the food sources for the dinosaurs died back dramatically and hence the dinosaurs were no longer fit. Selection did the rest.
There are, actually, still many missing pieces of this puzzle. The universality of dinosaurian extinction, except for the bird clade, would seem to require more explanation than this asteroid impact scenario can convincingly supply. There are competing or at least supplemental hypotheses. Many species of mammals and birds also went extinct. But no dinosaurs other than birds survived. Several hypotheses have been advanced to explain this phenomenon as well. Among them have been the general average sizes of birds and mammals (at the time there were no mammalian megafauna to speak of since all of the large animal econiches were occupied by dinosaurs), which were much smaller than the average dinosaur and that size difference plus metabolic energetics (e.g. body temperature maintenance) gave smaller birds and mammals an advantage (somehow). Other hypotheses involve the fact that most mammals, at the time, were burrowers so might have been protected from the harsher aspects of the climate changes.
Two Major Evolutionary Inventions
All of these hypotheses may work as explanations for mechanisms that helped provide some kinds of fitness to some species. And all of them may help explain a little piece of the phenomena. But there is another possibility that would be more universal for the bird clade and mammals that hinges on two completely new attributes for both groups. One of those explanations hinges on the increased capabilities of brains to process complex models of the environment, a competency shown by both birds and mammals but currently believed to not be the case for non-avian dinosaurs. The other is a more subtle competency that is just now gaining a greater understanding in the theory of evolution and that is the recognition of evolvability as a factor in the resilience of a species in the face of environmental stresses. I have mentioned this phenomenon in other posts.
The first factor involves an animals's behavior to be based more on learned relations with the environment than on instincts. We are still learning about the learning and conceptualizing capacity of birds (see: bird intelligence) but it now appears that many types have much higher cognitive abilities than we had assumed. The mammalian brain is evolved to rely on learning and memory as a major factor in shaping behavior. The cerebral cortex is greatly expanded over what we find in modern reptiles. From studies of dinosaurian brain cases (endocasts) it appears that some dinosaurs may have been evolving more cortex which would suggest they were becoming more intelligent than the general reptiles. But the avian line had already evolved a much larger cortex by the time of the End-Cretaceous event. Aside from flight and generally smaller sizes, what demarks the avian sub-clade is their ability to learn and remember.
The earliest mammals were already showing this expansion of cortex and learning ability. They were likely quite adaptive having to contend with the dominant dinosaurian clade kept them on their toes, so to speak. The key to resilience in both birds and mammals depended on their ability to learn and adapt to changing conditions within broader limits than the dinosaurs and all other genera.
Learning and adaptation, however, were not sufficient to deal with the kinds of radical changes that must have come with the Cretaceous–Paleogene (K-Pg, formerly called the K-T for Cretaceous-Tertiary) event. Increased cortical processing power was an initial step in the right direction but too many biological constraints would prevent specific animals from changing their behaviors too much. For example most birds and mammals have a limited range of foods that they can take advantage of. Nevertheless many species show an ability to eat a wider range of foods as compared, for example with fish or reptiles.
But another much less appreciated or understood aspect of evolution has been at work for a long time to help living systems become much more resilient than they would have been under a strictly neo-Darwinian scheme, i.e. random mutation of protein coding genes and natural selection. Evolvability is a property of a species to generate variations that are, in a sense, potentially pre-adaptive. That is, they can increase the rate of mutations, especially in selective genes, that increases the possibilities of advantageous results. This math works best for large population sizes or populations that are growing rapidly. There have to be many more individuals being conceived so that the increased mutation rate doesn't swamp the fitness of the species. This is because, as most people know, mutations are more often deleterious than helpful. There has to be an ability to “waste” individuals that end up with destructive mutations.
On the other hand, the promotion of mutations in selective genes may not actually produce a preponderance of harmful mutations. There are classes of proteins, the targets of genetic codes, that are able to take on many alternate sequences of amino acids without overly changing their basic functions. Indeed some new sequences may bestow completely new functions for the proteins without necessarily diminishing their potency in their primary functions. It seems that there are a large number of genes that are accompanied by DNA segments that “promote” mutations in those genes, perhaps by some kind of epigenetic mechanism. Those DNA segments may be triggered into action by environmental stressors that work through second and tertiary messengers to alter an epigenetic factor such as methylation of that segment. This is all still speculative in terms of exact mechanics, but the logic is beginning to look solid.
Evolvability increases the likelihood of finding a viable genotype/phenotype representative (or actually a small subpopulation of them) quickly in a search of variation space. It is a quick turning on of a higher mutation rate but with a higher probability of producing adaptive variants. Clearly, such a mechanism would be advantageous to the possessors in times of high stress. It turns out that many clear bottleneck events, where the number of individuals is drastically reduced during a radical environmental change, may be explained by the possession of higher evolvability by the species involved.
Evolvability has been evolving. It was actually recognized in a primitive form in bacteria that could adapt to restrictions on nutrients that they had evolved to require in their environments. Deprived of a key nutrient that they could not normally manufacture, but where the precursor components were available, evolved into a population that gained the ability to synthesize the needed nutrient. Not much work had been done on exploring this phenomenon in multicellular organisms as it was barely understood in simple bacteria. But recent developments in understanding the nature of the genetic expression control network of DNA that was previously thought to be “junk” have revived an interest in this. Finding the epigenetic mechanisms along with small RNA molecules that modulate various stages of gene expression, from transcription to interfering with protein construction at the ribosomes (even acting as enzymes — ribozymes — to control proteins directly) in development (see: Evolutionary Development) as well as in response to stresses has opened up some new possibilities. It has given us some attractive ways to explain many curious phenomena in evolution, such as punctuated equilibria. The key is to recognize that mutations do not just affect protein-coding genes but can affect the DNA in the control network as well. Mutations in, for example, a gene expression regulator (see especially: functions of non-coding DNA) could cause a radical but non-lethal change in morphology, the “hopeful monsters” theory of sudden changes in species.
We now have a pretty clear picture of evolution of these control networks even though we haven't yet worked out the details of control. The human genome project really opened everyone's eyes. The question was how could the most complex and intelligent being on the planet have fewer coding genes than animals and plants lower on the phylogenetic tree (only between 20 and 25k protein coding segments)? The answer is that it isn't the number of different proteins that are in the phenotype, but rather the way in which those building blocks and enzymes are used in different tissues, the timing of when they are expressed during development and so on. In other words what makes animals more complex is variations in the control of gene expression. For example my own hypothesis regarding the development of larger Brodmann area 10 patches in human brain evolution, leading to sapience (or presapience) involves some subtle changes in when and for how long certain genes that lead to bigger BA10 as well as possibly changing the cytology of the patch resulted from probably few mutations in the DNA involved in controlling the expression of important genes in brain development.
Now evolvability has more opportunity than just promoting key genes to mutate. It can work by promoting mutations in the control circuitry as well, leading to many more possibilities in generated variation. It turns out that birds and mammals enjoyed an explosion of the use of control circuits in DNA meaning that they might have had this greater advantage compared with the dinosaurs. We can't know what sort of control networks in DNA the dinosaurs sported. But if there is a relation between the extent to which control networks generate greater evolvability in birds and mammals as compared with dinosaurs and the degree to which the brains of the former two groups had exceeded the latter, then we might have a possible answer to why the former survived and the latter didn't. Birds and mammals were actually poised to deal with the cataclysm of the K-Pg event both in terms of greater behavioral adaptability and evolutionary adaptability. The latter did not require the millions of years we ordinarily associate with evolution. Higher evolvability and resulting saltation would have rapidly produced variants that were more fit. Furthermore, as conditions settled down from the worst just after the event, these mechanisms could explain rapid evolution to fill the developing econiches in adaptive radiation.
And that brings me to considerations for humanity's future going into a likely bottleneck event of our own making. As the understanding of EvoDevo and the human genome deepens it seems the evidence points ever more strongly toward the idea that we are a super-evolvable species! We have actually seen rapid evolutionary developments in the human species over the past 20- to 100 thousand years of our existence. I've written elsewhere about these so will not dwell on it. The bottom line is that the future of humans may not be as bleak as the more common beliefs would have it.
First consider that the natural variation in our populations for traits such as intelligence and sapience is already pretty high. Humans have been under mental stress since the invention of agriculture, which means what evolvability we have has already been at work for at least 10k years. There are some in our very midst who are pre-adapted for the kind of radical changes we expect to see before long. And they haven't been sitting idle waiting for the worst to hit. There are many people who seem to me to have higher than average sapience who are already considering what they have to do to survive and thrive even in the nightmare scenarios we've come to expect. Also, it is not likely that they are going to expose their plans so you may never hear of them!
I fully expect that a race of highly sapient individuals will go into and emerge from the bottleneck and provide the seed for a new species (or even more than one!) of eusapients someday. Earth isn't over. Nor is sentient life on this planet. Life evolved for more than 3½ billion years and as it did, it learned a few handy tricks from a wide variety of past cataclysms. Evolvability has improved over that time. I feel pretty comfortable with letting nature takes its course in the future. It isn't over for the planet and it isn't over for the naked ape.