*Note: this article has been updated since its original posting as more supporting info emerged and incorporated into the ongoing series on alternative evolution.
Well, if it didn’t happen by Neo-Darwinian means, how did evolution occur?
How Reptilian are YOU?
I am sure we have often heard the expression that you have a reptilian brain. This comes from a literal interpretation of a theoretic concept developed in the 60s by American physician and neuroscientist Paul D. MacLean and as Wikipedia state: he “propounded it at length in his 1990 book The Triune Brain in Evolution’ ” If you look it up, you will see why he has a lot to answer for in mythologising this meme as it was he, who coined the terms ‘R’-complex/reptilian complex. This is a misnomer and only reinforces the concept that we descended directly from some reptilian stock.
Source: Scientific America blog
The real issue is our over simplified idea of linear and direct common descent. For instance, below is an article that explains the misconception of direct evolution from reptiles and it also highlights the problem of terminology as to when mammals became mammals and I would just like to say: WHY DO WE KEEP CALLING THESE PRIMITIVE MAMMALS – MAMMAL-LIKE-REPTILES???
In many respects, the pelycosaurs are intermediate between the reptiles and mammals, and so they have commonly been referred to as “mammal-like reptiles”. The pelycosaurs indeed resemble large lizards in their overall appearance, but as we have seen, this is a misnomer since pelycosaurs are not reptiles.
The following brain study in vertebrates is by A.B. Butler, a Professor Emerita (retired) in the Molecular Neuroscience Department in the Krasnow Institute for Advanced Study. The quote is taken from ‘Evolution of Vertebrate Brains’:
The earliest mammals appear in the fossil record slightly before the earliest reptiles. Rather than thinking of the common ancestor of all amniotes as a stem reptile, which implies reptilian structures in the brain as well as elsewhere, it is correct to think of
the common ancestor as a stem amniote. Some of the most salient features of the brains of sauropsids (reptiles and birds) and those of mammals represent divergences from the common ancestral condition rather than sequential evolution of either the extant mammalian or sauropsidian condition to the other.
Butler (2009, 64)
…That brain enlargement and elaboration has occurred four times independently presents a very different reality of how brain evolution has operated than is perceived in the widely held folk-belief…
Butler (2009, 57)
Just as mammals diverged from the stem amniote stock, so did the sauropsid line that gave rise first to reptiles and, subsequently, birds, which actually are a specialized group of reptiles, just as are the other extant groups of this major taxon. In fact, recent genetic analyses have demonstrated that the thecodonts (crocodiles and birds), along with turtles and the rhynchocephalian lizardlike animal Sphenodon, are clustered as a monophyletic group, while the squamates (lizards and snakes) constitute the outgroup to them.
Butler (2009 64 and 65)
The structure of the brain is essentially a colony of stem-cells that have been programmed according to environmental conditions during evolutionary development to differentiate into neuron cells and wire together because they fire together – Hebb’s law and form complex networks of chemical firing – think of the neural net.
Hebb’s law has had recent verification as seen in Science Daily an article entitled: ‘Scientists control rapid re-wiring of brain circuits using patterned visual stimulation’:
“Hebbian Theory,” named after the McGill University psychologist Donald Olding Hebb who first proposed it in 1949 has been confirmed in real-time experiments as reported in a science paper on neurology (2014) and confirmed the axiom: “Cells that fire together, wire together. Cells that fire out of sync, lose their link.”
– (Science Daily May 28, 2014)
The brain itself appears to have evolved as a system for increased complexity and starts off as a bundle of nerve ends in a clump attached to a notch-chord – present in all vertebrate larval/embryonic forms (a basal chordate in its most primitive form). As development continues, the whole organism grows in relations to the whole system – it depends upon how much evolutionaary complexity potential that organism has to begin with.
The complexity comes from the connections and wiring of the neural network, which itself has evolved to different levels within vertebrate species. Think of the internet and all its hubs of communication. Well, the brain is all about how complex the wiring is. And, seemingly, the evolution of the universe isn’t that different to the evolution of the galaxies and planets and life itself – including brains and the brains that created and now use the world-wide web – the internet.
SPOT THE DIFFERENCE?
One image is of the neural network of a mouse’s brain, the other: simulated view of the universe from all the data we have gathered. There is something universal going on for sure. See recent article in this series.
Coming back down to earth, I’ll return to the brain itself and outline a few ways that it may have come to evolve independently at least four times in vertebrates as the brain study by Butler (2009) explains. The development of the brain is probably related as much to metabolic evolution driven by natural laws of growth/form and efficiency (again see other articles on this site relating to scaling laws etc), as it is to what an organism ate during its evolutionary journey. (See the aquatic ape theory – a tribute to Elaine Morgan on this blog) https://diggingupthefuture.files.wordpress.com/2014/07/tribute-to-elaine-morgan-by-maria-b-o-hare-diggingupthefuture.pdf
As discussed many times in this blog, Mother Nature has many ways to epigenetically express existing genetic code and therefore great variations on the same theme. Epigenetic evolution is basically a dynamic means of changing traits and characters over evolutionary timescales via a molecular process of cellular and chemical signaling, an interaction between any organism and its environment – particularly as it goes from a less defined primitive organism to a more defined and specialised species (this concept is based upon older principles of evolutionary development and supported by more recent molecular evidence). (see the series on: If it didn’t happen by Neo-Darwinian means, how did evolution occur?) Epigenetic evolution leads to different programs for that organism depending upon its evolutionary developmental experiences and inherent degree of complexity. Nature therefore does not put all her eggs in one basket so to speak.
The point being, epigenetics can, according to environmental conditions, reprogram the genetic code by altering the way the genetic code is expressed. As many studies show, epigenetic processes driven by environmental factors can rather radically alter an embryo. By extrapolation, an embryo-like or evolving (developing) species may be epigenetically expressed as a dramatically different species to how it started off. (Note that evolutionary development has settled down these days as the species seem to be in their more matured adult form – less morphing than in earlier times judging by the research investigated throughout these blogs).
Basically, in the past it was a bit like asking: how do you like your eggs? And the answer would be: Well, it depends on the weather of course. Sorry about the bad attempt at humour and especially if you don’t get it – but it will make more sense if you have read a range of articles on this site. Returning to the misguided assumption of mammals and reptiles being directly descended from a reptilian type ancestor, this is what Butler states:
… reptiles did not give rise to mammals any more than mammals gave rise to reptiles. In regard to embryological development, it likewise generally proceeds from the general (common ancestral features) to the specific (specializations of the taxon) […]. What is clearly established is that all taxa have their own specializations. Each taxon has a mix of primitive features.
Butler (2009, 64)
Now taking the brain study above and many other lines of investigation to their rather different conclusion of: going from an ancestral condition more than simplistic linear and direct descent, we should perhaps refer to ancestral features as common traits converging on an efficient specialism for the fundamental forms of animals. Primitive features that were once shared by all amniotes and independently evolved and diverged as vertebrate animals. When reviewing the fossil record in this different light, it is amazing how it all starts to make more sense. It really does look like generic generalist and primitive forms of land-walkers (amniotes and tetrapod types) began to specialise and present less primitive and more defined features of mammalian, reptilian, lizard-type and avian species forms. This less literal form of common descent (not forgetting the great web of life via common evolutionary hybridisation/horizontal gene-transfer/mobile genetic elements remodelling genomes, and microbial mergers of ways of producing genetic novel traits and orchestrated via epigenetic expression or differential expression of the same and existing genes as outlined in several articles on this blog), can, I believe, be ultimately explained by bringing into the evolutionary picture the fundamental metabolic form of fish, amphibian, reptile/lizard, bird, mammal from generic forms.
For instance, even though fish are vertebrates, they may never have walked or lived out of water (as discussed in other articles on this site – see why fish can’t grow fishy fingers for example), so as the brain study also suggests, based upon the evidence, we may be looking at a common ancestral condition and the different levels of evolutionary complexity being driven by inherent metabolic network complexity of a given species. Going from the generalist or generic form – common ancestral condition of a stem amniotic (egg-laying) to a diverse range of very primitive walking tetrapods who could activate full limbs and digits? Tetrapods who continued to exchange their genetics via interbreeding, where simpler tetrapods began specialising earlier due to their correspondingly less complex metabolic potential, and conversley, more complex metabolic systems found within specialising mammalian forms may have taken much longer to fulfill or continue to expand their evolutionary journey and ultimate speciation (specialist form).
Speciation may have therefore gone from an ancestral condition to more specialised species forms in a non-linear pattern of radiant diversification and specialisation according to fundamental forms of amphibian (stabilising earliest, quickly followed by simultaneous stabilisation of lizard, and both primitive fundamental types of reptile and mammalian forms. Later diversification of more complex warm-blooded and complexly wired brains of the reptile form may have led to avian (bird development of some). Similarly, some inherently more complex mammalian forms may have had a much longer evolutionary path and only fully specialised after a developing and diversifying as generic higher primate forms.
The present-day developmental modes of basal amniotes – who evolved seemingly, in line with evolution metabolic complexity and specialism, reveals quite a lot about the possible evolutionary route/modes of development of the past. Take for example, marsupial animals do a sort of in-between mode of development – they keep their young in a pocket/pouch for a while. So they are not quite egg-layers or live-birthers. All creatures with four legs and walk on land, whether upright or otherwise (tetrapods) seemed to have laid eggs at some point in their primitive evolutionary development. This is a very special egg (amniotic egg built for the long haul) and it can be laid, kept inside forming a placenta or kept in pouch as mentioned bove and did you know, there is such a thing as an egg-laying mammal?
Let us now look at a particularly strange mammal.
These critters known as monotremes illustrated above, are egg laying mammals. They are strange indeed, particularly the platypus types as it really doesn’t look like it has made up its mind what it is. These are very primitive types, as far as mammals go and have many features in common with reptile forms apparently. This is because of our terminology of using reptile anything as a reference to any primitive features. This is based upon a presummed common ancestor of mammals and reptiles. And as that ancestor is assummed to more reptilian in its primitive form (as of course mammals only evolved long after the reptiles as we are often told), then we are going to end up with everything that isn’t fully specialised as a species – referred to as reptilian-like mammal or a mammalian-like reptile and we are left with an entirely false interpretation of the fossil evidence.
What seems to be a fundamental distinction between reptiles and mammals is not that most mammals give birth to live young rather than lay eggs, except for those strange primitive type mammals such as monotremes, or the rearrangement of some bones (epigenetic re-coding would also cause such changes in bones, eggs, development, brains and just about everything else), it is their metabolism. and energy exchange with the environment that would appear most fundamental. I.e., warm-blooded or cold-blooded, which is fundamentally different. Their brain wiring is also fundamentally different which may be due to their less complex metabolism compared to even primitive mammal forms such as the monotreme types.
The link between temperature (environment) and metabolism and therefore brain complexity and size helps explain how brains could have evolved independently in vertebrates without having to go through a literal descent from a reptilian ancestor, is indicated in the next study. It should help explain HOW EVOLUTION of the BRAIN may have happened if mammals didn’t descend directly and linearly from a reptilian ancestor, but instead, from a common ancestral condition:
The brain size study and temperature:
Source: Figure 2: The natural logarithm of body mass-corrected relative brain size vs. inverse temperature in vertebrates.
Relative brain size is expressed as a percentage of body mass, body mass is expressed in grams, and temperature is in degrees Kelvin (T)
The tremendous variation in brain size among vertebrates has long been thought to be related to differences in species’ metabolic rates. It is thought that species with higher metabolic rates can supply more energy to support the relatively high cost of brain tissue. And yet, while body temperature is known to be a major determinant of metabolic rate, the possible effects of temperature on brain size have scarcely been explored. Thus, here we explore the effects of temperature on brain size among diverse vertebrates (fishes, amphibians, reptiles, birds and mammals). We find that, after controlling for body size, brain size increases exponentially with temperature in much the same way as metabolic rate. These results suggest that temperature-dependent changes in aerobic capacity, which have long been known to affect physical performance, similarly affect brain size. The observed temperature-dependence of brain size may explain observed gradients in brain size among both ectotherms and endotherms across broad spatial and temporal scale.
That’s all folks until the second last part in the series next week which will deal specifically with why fish can’t grow fishy fingers and may never have walked. It will also offer an evolutionary alternative for how all these stem-amniotes got onto land in the first place if it wasn’t due to walking fish.