In the preceding two posts on the biomimetics of nervous system evolution (check out Part I and Part II here) we looked at the basic concepts of bio-inspiration, new computational methods for biology and design. We also touched on ideas from the complexity sciences that relate both to biological and technological systems.
Now, it’s time to combine these insights from theoretical biology, comparative genomics and bioinformatics to reach an updated view on the convergent emergence of nervous systems during animal evolution.
This post deals with the evolutionary kinship relations determined by genomic sequence information (phylogenomics) of the five main animal groups, the distribution of nervous systems among them and the surprising way bio-complexity, instead of gradually arising and evolving over time, emerges suddenly and is already highly organized.
“Eventually, we are going to see that there is no single route to bio-inspired AI. Rather, the opposite is true - a plethora of ingenious, convergent solutions that more or less emerged simultaneously in nature.”
Eventually, we are going to see that there is no single route to bio-inspired AI. Rather, the opposite is true - a plethora of ingenious, convergent solutions that more or less emerged simultaneously in nature. Functionally equivalent (but molecularly distinct) instantiations of nervous systems across animal complexity are paving the way to future tailor-made technological applications that fulfil specific purposes in different cognitive contexts.
The Five Phyla
The metazoan or animal phylogenetic tree of life contains five major branches (‘phyla’ in systematics lingo, Figure 1). Let’s start with the four ‘simple’ groups of life forms called lower metazoans or pre-bilaterian organisms: Porifera (sponges), Ctenophora (comb jellies), Placozoa (made up by only one species, a flat, one-cell layered creature) and Cnidaria (jellyfish and medusa). The fifth branch is made up by Bilateria. Most animals, including vertebrates (fishes, frogs, birds, humans), arthropods (insects, spiders, crustaceans), and molluscs (snails, mussels, squid), are part of the group Bilateria.
A single median axis divides the body into equivalent halves that are mirror images of one another. These halves run from the front end, which is usually differentiated into a head, to the back end, often in the form of a tail.
This body plan contrasts with that of radially symmetrical animals like jellyfish and sea anemones (Cnidaria) which have multiple similar parts arranged around a central axis, so their bodies lack a front and back end. The bilaterians comprise 99% of all known animal species and are the morphologically and behaviorally most complex metazoan group.
Figure 1: Simpliﬁed phylogenetic tree of eukaryotes.
Porifera, Placozoa, Ctenophora, Cnidaria and Bilateria together form the Metazoa (dashed lines denote unclear phylogenetic position). Extant nervous systems, i.e. ones prevalent in species alive today, can be found in ctenophores, cnidarians and bilaterians. The origin of Metazoa can be approximately dated to 780–800 Million years ago (Mya) and the cnidarian/bilaterian LCA to ca. 700 Mya. The origin of Bilateria is approximated at 688 Mya (not shown). Red dots indicate convergent complexiﬁcation events in early nervous systems occurring in the respective lineages.
Three of the five phyla mentioned above contain species with nervous systems of varying complexity and organizational sophistication: in ctenophores and cnidarians neurons are organized in a net-like structure (nerve net) while bilaterians display centralized nervous systems and brains (centralization of nerve nets itself being a convergent trait within this group). Sponges and placozoans are lacking discernible neural features altogether.
Does the nervous system in the Metazoa have a singular origin or did it emerge independently in comb jellies, cnidarians and bilateral organisms?
Establishing a Sequence of Events
In order to establish the entire sequence of events in early nervous system evolution, determining the correct branching order of the main metazoan phyla is essential.
Up until a few years ago, a consensus view among evolutionary biologists suggested that sponges branched off first, followed by either ctenophores (that are equipped with neural nets and generally have a pretty complex tissue organization) or placozoans (with a secondary loss of neural structures depending on their actual position). Lastly came the split of the closely related ‘sister groups’ Cnidaria and Bilateria (the former having a nerve net and the latter centralized nervous systems).
“Although certain homologies, i.e. traits originating from a common ancestor, have indeed been detected in the nervous systems of Cnidaria and Bilateria, it is far from certain if the nervous systems in these two groups share a common origin.”
Since Cnidaria and Bilateria form sister groups, much attention in terms of genetic neurodevelopment has been given to the last common ancestor (LCA) of these two clades (Figure 1: LCAC/B). Although certain homologies, i.e. traits originating from a common ancestor, have indeed been detected in the nervous systems of Cnidaria and Bilateria, it is far from certain if the nervous systems in these two groups share a common origin.
Considerable debate surrounds the recent claim that, actually, Ctenophora might be the most basal, i.e. first branching, metazoan group of organisms. One vocal proponent of this claim has been the Russian-American scientist Prof. Leonid Moroz of the University of Florida, who became one of the first genomics experts to sequence the DNA of the elusive comb jellies .
To achieve this marvelous feat he even installed a cumbersome sequencing machine onboard of a research vessel, since comb jellies literally dissolve into water once they are taken out of their native habitat. Therefore, the samples have to be processed and sequenced immediately because it’s too late for experimentation once they are brought back to the lab onshore.
“Why would any group of organisms lose such a type of intricate structure that was certainly able to give a competitive edge in the merciless struggle for existence that was the ancient oceans?”
If the morphologically complex comb jellies actually branched off first after the origin of animal multicellularity, this would suggest – assuming deep homology of the origin of nervous systems – two loss events of neural structures in Porifera and Placozoa. But why would any group of organisms lose such a type of intricate structure that was certainly able to give a competitive edge in the merciless struggle for existence that was the ancient oceans?
The Possibility of Convergent Evolution of Neurons and Nerve Nets
Another tantalizing scenario is presented by the possibility of convergent evolution of neurons and nerve nets in the stem line of Ctenophora and the one leading to the LCAC/B with primary absences of neurons in Porifera and Placozoa.
This question is called the ‘Polygenesis (Convergent Evolution) vs. Monophyly (Singular Origin)’ debate, and it has profound implications for how scientists view the evolutionary process. The emergence, evolution and development of early nervous systems is a field of research ripe with competing views on the basic premises framing the discussion, the branching pattern of lower metazoans being one of the foremost points of contention.
The following statement by comb jelly expert Prof. Moroz encapsulates the essence of this series about convergent evolution and biomimetics:
“What everyone has said about complexity is wrong. It can happen more than once.” 
Figure 2: Fact Sheet on the enigmatic comb jellies (phylum Ctenophora). Credit: Dr. Ross Piper.
The unsolved polygenesis vs. monophyly mystery relating to the emergence of neurons and nervous systems, which centres on the branching position of Ctenophora, serves well to illustrate a general problem in evolutionary biology.
The Evolution of Complex Traits
Identifying patterns in the evolution of complex traits is often difficult as they display a mix of shared and independently derived features. For example, sophisticated sense organs such as image-forming eyes evolved on many independent occasions during the Cambrian. Incorporating pre-existing photoreceptors, image-forming eyes evolved independently in arthropods, cephalopod mollusks and vertebrates.
Eyes, however, were not built from scratch every time. Rather, they arose by the modification of pre-existing gene regulatory circuits already established in early metazoans. Based on the presence of highly conserved ancient developmental pathways, generative processes and conserved molecular signaling cascades underlying cell-type specification, researchers assume that the same outcomes in independently evolved complex traits can be partly explained by ‘deep homology’.
Nevertheless, deep homology of shared developmental toolkits is not sufficient for explaining the repeated and independent emergence of complex systems. This follows from the realization that specific biological systems (for example nervous systems) cannot exhaustively be described solely in terms of their underlying gene regulatory network.
“Convergent evolution of complex traits goes beyond the iterated deployment or repurposing of shared developmental building blocks in independently evolving lineages.”
Convergent evolution of complex traits goes beyond the iterated deployment or repurposing of shared developmental building blocks in independently evolving lineages. It must be distinguished between the origin of highly conserved genes, the formation of developmental gene regulatory networks that control morphogenetic patterning and the subsequent origin of phenotypic complex traits.
For an excellent review on the evolutionary history of such “complex homology” in early nervous systems, check out the following non-technical review paper by convergence expert Dr. Benjamin Liebeskind from the University of Austin. The authors argue that it might be helpful to organize and understand the evolution of animal forms “as a set of stable states toward which the different lineages have been pulled.”  Further, regarding explosive systemic changes in early nervous system evolution, the authors note:
“Around 600 Mya, after all five major lineages (ctenophores, sponges, placozoans, cnidarians, and bilaterians) had diverged, there was a sudden change. Large expansions of the gene families associated with synaptic and electrical complexity occurred together with profound changes on the biophysical level. These changes occurred convergently in the stem lineages of ctenophores, cnidarians, and bilaterians.
“Although many of these genomic events cannot yet be dated precisely, they most likely followed the end of the worldwide glaciation events, contemporaneously with the rise of oceanic oxygen and macroscopic animal forms. The rise of inter-animal predation in the early Cambrian probably provided selective pressure to evolve complex behaviors, neural organization, and musculature.” 
Naturally, all living animal lineages have had the same amount of time to evolve since the most recent common ancestor of all animals. Traits have been repeatedly and independently gained and lost. Still, a major transition in the organization of nerve nets has occurred in the stem line of Bilateria, enabling a vastly expanded phase space of information processing and adaptive behavior based on learning and memory.
Gaining ‘Functional Closure’
Evolutionary dynamics active during evolutionary macrotransitions – such as the origin of life, protocellular evolution and the emergence of multicellularity – might differ fundamentally from gradualist and strictly selection-driven models of evolution.
Sudden and systemic increases in bio-complexity happen through what Prof. Stuart Kauffman, one of the leading experts in the field and a complexity theory veteran in his own right, has dubbed ‘functional closure’: Novel organizational functionalities in complex biological networks arise via the intersection of previously unrelated biomolecular pathways.
In part IV of this series we are going to explore the ecology of the Neoproterozoic era, the time when nervous systems first emerged in the ancient oceans more than 600 million years ago. We will further differentiate between the functional peculiarities of information processing in Ctenophora, Cnidaria and Bilateria. Finally, the last two chapters of this series will be dedicated to deriving new technological applications in the fields of AI and robotics from the insights we have gained so far by looking at Nature’s way of patenting.
 Moroz, L. L., et al. (2014). "The ctenophore genome and the evolutionary origins of neural systems." Nature 510(7503): 109-114.
 Evolution May Be Drunk, But It’s Serious About Making Brains, Nautilus Blog
 Liebeskind, B. J., et al. (2016). "Complex Homology and the Evolution of Nervous Systems." Trends Ecol Evol 31(2): 127-135.
When you hear the words evolution and technology in the same sentence, it’s tempting to think of cyborgs. Popular culture has been fascinated by human augmentation for decades, but as technology improves it is stepping out of the realm of fantasy and becoming real.
Take Neil Harbisson, a Northern Irish artist who is legally recognised as a cyborg by the government. In 2004, he had an antenna permanently attached to his head that allows him to hear and feel colours in the form of audible vibrations. Born colour-blind, the technology was a way for Harbisson to experience colour.
It’ll be a while before solutions like this see any kind of traction, but perceived eccentrics like Harbisson are pushing the envelope. Where will it lead?
Illustrations by Kseniya Forbender
To contact the editor responsible for this story:
Margarita Khartanovich at [email protected]
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