No, don’t be ridiculous

What a disgusting suggestion! Uck!! A criminal conviction for bestiality will earn you a thumping prison-sentence and an odium that will shrink your social life. Anycase, have you forgotten the text-book definition of a species? That’s right; key to the concept are that populations must be reproductively isolated. So no inter-specific hanky-panky, thank you very much. It is just as well animals don’t read the text-books because in reality species do interbreed (and you should see the plants …). Such hybridization occasionally opens new evolutionary doors, but more often the outcome of such miscegenation is sterility. And what about us, Homo sapiens? No question that we descended from other species, but now alone we proudly stand. Would we ever stoop to interbreeding? Perish the thought!

Yes, of course

Our genomes tell a very different story. We may be alone, but a little beyond historical memory we had close companions in the form of the Neanderthals and the more recently discovered Denisovans. Skeletons of the former abound and reveal a powerfully built customer. Given Denisovans are known from only a couple of teeth and a tiny finger bone, reconstructing their original appearance might seem a tall order. Let’s ask Holmes: “Observe, Watson, mere scraps, but the tooth, is it not somewhat archaic? Summon one of my Irregulars, I want this finger-bone to be taken poste-haste to that ingenious DNA lab in Leipzig …”. Ingenious indeed, because not withstanding almost invariably massive microbial contamination, original DNA is not only recovered but it confirms that although Denisovans, humans and Neanderthals are genomically near-identical there real differences in the DNA and they yield extraordinary insights. First, Denisovans and Neanderthals are the closer cousins (separating about 550,000 years ago), whilst we and Neanderthals went our separate ways perhaps a quarter of million years later. Did I say “separate”? Not quite. Lurking in our genomes are chunks of Denisovan and Neanderthal DNA and they didn’t get there by sneezing. The only explanation is interbreeding, in the trade this transfer is known as introgression. The story, however, is a bit more complicated. Writing as a Eurasian roughly 2% of my DNA is Neanderthal. Amongst Asians this figure is slightly higher, but if we look at sub-Saharan Africa then with the curious exception of the Maasai only a trace of this imported DNA can be found. So did they escape introgression? Not at all; sub-Saharan people carry a tell-tale signature of a separate encounter with another archaic hominin, and perhaps as recently as 35,000 years ago. The Denisovan story is if anything more intriguing. In the inhabitants of Papua New Guinea about 6% of the DNA is of Denisovan origin and appreciable amounts are also found in surrounding populations. Whilst these genomic footprints provide unequivocal evidence of interbreeding, successful hybridizations were still extremely rare. One estimate suggests one successful mating roughly every eighty generations will suffice to ensure the transfer of Neanderthal or Denisovan DNA.

It all depends on the question

So does this introgressed DNA make us any less human? Not at all! First of all humans readily interbreed. Whether or not you are the proud carrier of Neanderthal or Denisovan DNA makes no difference at all. We are one species. Note also that the original quantities of imported DNA were probably greater, but some of the visitor DNA has been shown to the door. It is no coincidence that in this respect the X-chromosome is notably “clean”; on this sex-chromosome introgressed DNA might do real damage. Elsewhere, however, there might be real advantages, although deciding exactly how is more tricky. First, by no means all living human populations have the same profiles of introgression so any benefits cannot be universal. Second, genomic history is complex; in some cases the DNA may actually derive from an older common ancestor. This is important because features such as language might significantly predate the emergence of Neanderthals, Denisovans and ourselves. Third, genes are usually multifunctional and it is risky to say gene A does only function X. Even so there are some interesting links. Most striking, perhaps, is the clear connection between the remarkable adaptability of Tibetans to high altitude-life and what is clearly a Denisovan import. To be sure other links are more tenuous, but they may include skin colour (and hence UV protection or vitamin D synthesis), skeletal structure, the immune system and cognitive capacities.

Details of when, where and how often these hominin species met are sketchy, but the introgressed DNA we carry is mute testimony to a far more complex history of distributions, contacts and migrations than was once imagined. And as the unexpected discovery of the “hobbits” on Flores (at conferences they are called Homo floresiensis) should remind us, maybe there were still other early hominins roving the planet at the same time, yet to be discovered? Remember that currently the Denisovans are known from only a single Siberian cave. To explain, however, where the remnants of their DNA are now found can only be explained if originally they had a huge geographical range, rivalling that of the Neanderthals. The Denisovans are yet to tell their full story, but in the case of the Neanderthals a persistent idea is that despite their success ultimately their extinctions were the result of coming second-best, runners-up against the smarter us. Maybe so, but evidence from diets, cooking, hunting, clothing, technology and most importantly cultural and symbolic activities suggest that it could have as easily been a Neanderthal writing these lines. Maybe their fatal flaw was tiny populations that were too vulnerable. Even so, Neanderthals possessed speech … and imagination. Forever gone, now they haunt our imaginations. It is an early summer day near the River Jordan and mist floats across the Palaeolithic meadow. Was it an accident or already arranged? Either way Neanderthal and human, male and female meet. Boy meets girl as they say, but this time across the species boundary.   50,000 years later we still carry a clear genetic memory of this fleeting meeting.

Text copyright © 2015 Simon Conway Morris. All rights reserved.

Further reading

Abi-Rached, L. et al. (2011)  The shaping of modern immune systems by multiregional admixture with archaic humans. Science 334, 89-94.

Ding, Q-L. et al. (2014)  Neanderthal introgression at chromosome 3p21.31 was under positive natural selection in east Asians. Molecular Biology and Evolution 31, 689-695.

Fu, Q-M. et al. (2015)  An early modern human from Romania with a recent Neanderthal ancestor. Nature 524, 216-219.

Gokhman, D. et al. (2014)  Reconstructing the DNA methylation maps of the Neanderthal and the Denisovan. Science 344, 523-527.

Hardy, B.L. et al. (2013)  Impossible Neanderthals?  Making string, throwing projectiles and catching small game during Marine Isotope Stage 4 (Abri du Maras, France). Quaternary Science Reviews 82, 23-40.

Hawks, J. (2013) Significance of Neandertal and Denisovan genomes in human evolution. Annual Review of Anthropology 42, 433-449.

Huerta-Sánchez, E. et al. (2014)  Altitude adaptation in Tibetans caused by introgression of Denisovan-like DNA. Nature 512, 194-197.

Lachance, J. et al. (2012)  Evolutionary history and adaptation from high-coverage whole-genome sequences of diverse hunter-gatherers. Cell 150, 457-469.

Mendez, F.L. et al. (2012)  A haplotype at STAT2 introgressed from Neanderthals and serves as a candidate of positive selection in Papua New Guinea. American Journal of Human Genetics 91, 265-274.

Meyer, M. et al. (2012)  A high-coverage genome sequence from an archaic Denisovan individual. Science 338, 222-226.

Neves, A.G.M. and Serva, M. (2012) Extremely rare interbreeding events can explain Neanderthal DNA in living humans. PLoS ONE 7, e47076.

Overmann, K.A. and Coolidge, F.L. (2013) Human species and mating systems: Neandertal-Homo sapiens reproductive isolation and the archaeological and fossil records. Journal of Anthropological Sciences 91, 91-110.

Prüfer, K. et al. (2014)  The complete genome sequence of a Neanderthal from the Altai Mountains. Nature 505, 43-49.

Sankararaman, S. et al. (2012)  The date of interbreeding between Neandertals and modern humans. PLoS Genetics 8, e1002947.

Sankararaman, S. et al. (2014)  The genomic landscape of Neanderthal ancestry in present-day humans. Nature 507, 354-357.

Wall, J.D. et al. (2013)  Higher levels of Neanderthal ancestry in East Asians than in Europeans. Genetics 194, 199-209.

The old Jewish Cemetery; Venice, 1790. 

Goethe loosens the earth from the skull, and holds it up to the sun. 

Turning the fractured bone back and forth, he gasps.  A series of marks appear inside the cavity, reminding him of the vertebrae.  He has looked at this pattern many times, but without seeing what lights up before him today. 

Here is the shadow of a blueprint; the ‘primal repeating units’ of animal bodies, from which their many variations form. 

Puzzled, Götze watches his master’s eyes shine with delight. 

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Watch this baby babbling; her limbs move, often in time with her sounds.  the coupling of gestures and vocal calls are widespread amongst social vertebrates.  And the story began with fish.

The rhythm of our breath keeps us alive.  Conscious muscle movements are made through spinal nerve reflexes, but like our heart beats, the repeating sequences of muscle actions which fill and empty our lungs are outside our conscious awareness.

The movements behind repetitive activities like breathing are driven by rhythmic nerve impulses from ‘neural oscillators’.  These pattern-generating circuits, located in the central nervous system, are known as ‘Central Pattern Generators’.

To breathe, to speak and to swallow we use the same internal tube; that is our throat (the pharynx).  These activities are necessarily exclusive; consider what happens when a crumb ‘goes down the wrong way’.  Speech therefore needs to be coordinated with our breathing.

We make vocal sounds by passing air through the larynx as we breathe out, at the same time as vibrating our vocal folds (vocal cords).  These actions involve coordinating a sequence of repetitive movements inside the throat with the repeating muscle actions that drive our breath.

Communicating with sound evolved long before animals emerged from the sea onto land and began to breathe air.  Many fish use pectoral fin movements as communication gestures; some also generate sounds by fin waving.

In species of vocal fish, the calls are coordinated with these pectoral fin signals.  Significantly, the muscles operating these social communication cues are controlled using the same neural oscillator ‘module’.

‘Central Pattern Generators’ are neural oscillators that generate a rhythmic output, used to control repeating muscle movements.  These ‘neural metronomes’ were first discovered in insects, and produce their steady pulse without any sensory stimulus.  In contrast, our other nerves operate on a ‘stimulus-response’ basis.

 

Central Pattern Generators reveal what can be called our ‘deep homology’.  First discovered in insects, all vertebrates, including ourselves, have these ancient neural circuits.  They links vocal calls with gestures, and coordinate our ‘fins’ with our speech.

How do Central Pattern Generators work? 

We consciously control our limbs through spinal nerve reflex arcs.  In contrast, rhythmic movements controlling oscillating cycles are driven by Central Pattern Generators (CPGs).  These autonomous modules in the central nervous system produce a rhythmic output (like a neural ‘black box’).  CPG modules are comprised of a dense interconnected local network of neurons; a neural ‘node’.

These nodes are organised into three levels, each with a different function, and in each case, the parts of the circuit ‘higher’ in this organisation regulate the outputs of those below.

In the developing embryo there are functional units, ‘segments’ which give rise to our vertebrae and their associated nerves and muscles.  The nerves from each of these ‘segments’ form our local sensory spinal reflexes and also the CPG modules.  As needed, higher brain centres trigger these ‘neural motors’ to produce their rhythmical nerve impulses and drive all of our rhythmical movements from walking to chewing.

CPGs controlling rhythmic movements of the tongue, throat and breathing (including the vocal neural oscillator module) are in the lower brainstem and neck.  The CPGs that drive the rhythm of our walking are low down in the spinal cord, in the thoracic and lumbar regions.

Why don’t we sound like fish? 

Toadfish vocalise with either a sequence of repetitive grunts during aggressive encounters or the prolonged drone of their mating call.  In both cases, each nerve impulse from the vocal pattern generator produces a single synchronised contraction in their sonic muscles; this muscle pair flexes the rigid walls of the swimbladder, producing a ‘grunt’.  This sound receives no further processing.  As a result, its tone is rather mechanical.

Our voice, like that of frogs, birds and mammals, also begins with this simple rhythmic sound pulse.  This initial sound is then processed into croaks, calls songs and speech.  Our neck  allows us to create resonant areas in the throat which amplify certain frequencies.  Pitch is affected by vocal fold (vocal cord) tension, and manipulation of our tongue and lips produces precisely articulated words.

Why is ‘talking with our hands’ still a part of our language?

Many vocal fish make synchronised gestures with their front (pectoral) fins during mating calls.  These motor nerve connections from our ancient common ancestor are retained in other vertebrates.

People blind from birth move their hands when they talk.  Our vocal Pattern Generator circuits connects with both our larynx and pectoral muscles, coordinating our speech with our ‘body language’.  We subconsciously move our hands as we communicate thanks to these rhythmic central circuits.  As in all other vertebrates, we have inherited these from (as Palaeontologist Neil Shubin puts it) our ancestral ‘inner fish’.

Text copyright © 2015 Mags Leighton. All rights reserved.

References

Aboitiz, F. (2012)  Gestures, vocalizations, and memory in language origins.  Frontiers in Evolutionary Neuroscience 4, e2.

Bass, A.H. and Chagnaud, B.P. (2012)  Shared developmental and evolutionary origins for neural basis of vocal-acoustic and pectoral-gestural signalling.  Proceedings of the National Academy of Sciences, USA 109 (Suppl.1), 10677-10684.

Bass, A.H. et al. (2008)  Evolutionary origins for social vocalization in a vertebrate hindbrain-spinal compartment.  Science 321, 417-421.

Bostwick, K.S. (2000) Display behaviors, mechanical sounds, and evolutionary relationships of the Club-winged Manakin (Machaeropterus deliciosus). Auk 117, 465-478.

Bostwick, K.S. et al. (2010)  Resonating feathers produce courtship song. Proceedings of the Royal Society, B 277, 835-841.

Chagnaud, B.P. et al. (2012)  Innovations in motoneuron synchrony drive rapid temporal modulations in vertebrate acoustic signalling.   Journal of Neurophysiology 107, 3528-3542.

Dick, A.S. et al. (2012)  Gesture in the developing brain.  Developmental Science 15, 165–180.

Ghazanfar, A.A. (2013) Multisensory vocal communication in primates and the evolution of rhythmic speech. Behavioral Ecology and Sociobiology 67, 1441-1448.

Goethe, J.W. (1820).  Zur Naturwissenschaften überhaupt, besonders zur Morphologie; cited (p. 7) in G.R. de Beer The Development of the Vertebrate Skull.  Clarendon (1937).

Guthrie, S. (1996)  Patterning the hindbrain.  Current Opinion in Neurobiology 6,41-48.

Hanneman, E. et al. (1988) Segmental pattern of development of the hindbrain and spinal cord of the zebrafish embryo. Development 103, 49-58.

Iverson, J.M. and Thelen, E. (1999)  Hand, mouth and brain: The dynamic emergence of speech and gesture.  Journal of Consciousness Studies 6, 19-40.

Kelley, D.B. and Bass, A.H. (2010)  Neurobiology of vocal communication: mechanisms for sensorimotor integration and vocal patterning. Current Opinion in Neurobiology 20,748-53.

Marder, E. and Bucher, D. (2001)  Central pattern generators and the control of rhythmic movements.  Current Biology 11, R986-R996.

Shubin, N (2008)  Your inner fish; a journey into the 3.5 Billion year history of the human body.  Penguin Books Ltd.

Shubin, N. et al. (2009)  Deep homology and the origins of evolutionary novelty.  Nature 457, 818-823.