DESCENT FROM A COMMON ANCESTOR
The fact that all living organisms, from the simplest organisms to the largest modern mammals, contain cells that use the same basic chemical processes and pathways, and use the same genetic code, means that in this sense all living organisms are related. As David Christian points out, this can only mean one of two things: either life evolved only once, or it evolved more than once, but only one of those alternative life forms has survived to the present day, while all other lineages were eventually wiped out. “In either case, all organisms living today from humans to bananas to sea squirts and amoebae, are descended from the same bacterial ancestor”[1].
There are many animals that (more or less) closely resemble us, plants that resemble us a little less and on which we ultimately depend for our nourishment, and bacteria that resemble our remoter ancestors and to which we shall return to decay when our time is past. So it is that every one of the millions of species of animals on earth shares an ancestor with every other, and any two species are descended from an ancestral species which split in two[2].
Actually, it was Darwin who originally raised this idea. In 1837, he opened one of his notebooks and drew a rudimentary “Tree of Life” with a few simple branches, showing all life and all the myriad of species as having stemmed (literally) from a single ancestor, see below:
[1] Christian, ibid, 92.
[2] Dawkins, Greatest Show at 425. See also at 152,153, 187,188, 164, 312 and 298.
The fact that all living organisms, from the simplest organisms to the largest modern mammals, contain cells that use the same basic chemical processes and pathways, and use the same genetic code, means that in this sense all living organisms are related. As David Christian points out, this can only mean one of two things: either life evolved only once, or it evolved more than once, but only one of those alternative life forms has survived to the present day, while all other lineages were eventually wiped out. “In either case, all organisms living today from humans to bananas to sea squirts and amoebae, are descended from the same bacterial ancestor”[1].
There are many animals that (more or less) closely resemble us, plants that resemble us a little less and on which we ultimately depend for our nourishment, and bacteria that resemble our remoter ancestors and to which we shall return to decay when our time is past. So it is that every one of the millions of species of animals on earth shares an ancestor with every other, and any two species are descended from an ancestral species which split in two[2].
Actually, it was Darwin who originally raised this idea. In 1837, he opened one of his notebooks and drew a rudimentary “Tree of Life” with a few simple branches, showing all life and all the myriad of species as having stemmed (literally) from a single ancestor, see below:
[1] Christian, ibid, 92.
[2] Dawkins, Greatest Show at 425. See also at 152,153, 187,188, 164, 312 and 298.
He later developed this idea in Origin of Species, and did so without the benefit of the molecular evidence we now have, relying instead on the logic of his theory alone. “By the theory of natural selection”, he said, “all living species have been connected with the parent-species of each genus, by differences not greater than we see between the varieties of the same species at the present day; and these parent-species, now generally extinct, have in their turn been similarly connected with more ancient species; and so on backwards, always converging to the common ancestor of each great class. So that the number of intermediate and transitional links, between all living and extinct species, must have been inconceivably great. But assuredly, if this theory be true, such have lived upon this earth”[1].
With the aid of the full range of fossil molecular and other evidence now available to them, and assisted by computer programming which enables them to analyse tens of thousands of species at a time, scientists are now attempting to draw a tree of life that includes every known species, which will have about 2 million branches – perhaps more like a bush than a tree, as we shall see. Interestingly enough, animals and plants will take up only a tiny part of the tree, since most biodiversity on Earth is microbial[2].
Richard Dawkins recently described the common ancestor concept in his usual colourful phraseology before the Tree of Life project became a work in progress:
The common ancestor of people and budgerigars lived about 310 million years ago, and the ancestor that the duck-billed platypus shared with us lived 180 million years ago, nearly 3 times as long as the extinction of the dinosaurs. In both instances, the ancestral species split in two, and their two strands went their separate ways for the rest of time. The same ancestral species is shared by all mammals on one side of that early divide, and all reptiles (including birds) on the other[3].
Humans are not descended from monkeys, but we share a common ancestor with them. Nor are we are descended from chimpanzees, but molecular evidence shows that the common ancestor we share with them lived about 6 million years ago. Neither are humans are descended from lungfish, but we share an ancestor with them, and it is generally accepted that bats and humans also share a common ancestor. All birds have inherited their feathers from their shared ancestor, which had feathers, and no mammal is descended from that ancestor[4].
So, every animal is linked to every other animal by a chain of intermediates, let’s say by way of example rabbit to leopard since we are about to consider that relationship in a little more detail, and there are similar bridges from rabbit to wombat, from leopard to lobster, from every animal or plant to every other - including humans, and there is simply no evolutionary justification for the common assumption that evolution is somehow “aimed” at humans, or that humans represent “evolution’s last word”, or that man is higher than the other animals or species. It is nonsense to rank species on a ladder[5].
So a rabbit is not “just a rabbit” ...
Mention was made of the hypothetical link between rabbit and the leopard, via a chain of intermediates. When you look closely at each of those species, bear in mind that what you are now observing is not just a static being to which we give the denomination ‘rabbit’ or ‘leopard’. Each is, and continues to be, an evolving being. Now imagine that you are tracing, say, the rabbit back in time from the one you now see to its parent to grandparent to great-grand-parent, way way back in time until the rabbits just start to look a little different to the species we know today until it metamorphoses into an intermediate, let’s say something like a shrew. Then turn an evolutionary hairpin bend and move forward in time. For a while you will be following this shrew like creature until gradually it manifests itself into something more and more like a leopard and so on until we reach the species we know as a leopard today. All along the route on both sides there are numerous other offshoots we could have followed as different rabbit and leopard like species metamorphose into and from each other. We could just as easily have followed separate routes from porcupine to dolphin, from wallaby to giraffe, or from human to haddock[6].
That all sounds very interesting – even exotic perhaps - but there is a practical aspect to this with significant implications for modern medicine, which uses and experiments upon model systems to study more complicated ones, relying on the relatedness of all biological organisms, ourselves included. As Stuart Firestein points out: “It is the process of evolution, the mechanics of genetic inheritance and occasional mutation, that have conserved the genes responsible for making the proteins that confer electrical activity on neurons, as well as those that make kidneys and livers and hearts and lungs work the way they do. If that were not the case, then we couldn’t study these things in worms, flies, rats, mice or monkeys, and believe that it would have relevance to humans. There would be no drugs, no surgical procedures, no treatments and no diagnostic tests. All of these have developed using model systems ranging from cells in culture dishes to rodents to primates. No evolution, no model systems, no progress”[7]. Firestein himself was the recipient of a grant to study the olfactory system of the salamander, not because he had any particular fascinations with the salamanders’ sense of smell, but because they are an excellent model system for working out how brains direct molecules and how new brain cells might be generated and how smell plays a role on sex and production[8].
[1] Darwin, Origin of Species, Penguin, 2009, Chapter on the imperfections of the geological record, at 251.
[2] Carl Zimmer, “Scientists branch out to find missing links in the tree of life”, New York Times, reproduced in the Health and Science segment of the Sydney Morning Herald, 7 June 2002.
[3] The closest relatives of birds (whose family includes dinosaurs and snakes and reptiles) are to be found among the long extinct dinosaurs: Greatest Show, 160..
[4] The sources for this paragraph appear in Greatest Show at 425,152,153, 187,188, 164, 312 and 298
[5] Sources for paragraph: Greatest Show, 23-24, 158.
[6] This is an abbreviated form of the scenario depicted in Greatest Show at 23-25.
[7] Stuart Firestein, Ignorance – How it drives Science, OUP, New York, 2012, 72-73.
[8] Ibid, 75-76.
Is there in fact a “Last Universal Common Ancestor” (LUCA)?
It was Darwin who originally postulated the idea of a single common ancestor – “one primordial form” - from which all modern beings must have descended. In the fullness of time Darwin’s notion of universal common ancestry came to be supported by the fact that every organism, from microbes to human beings, uses almost exactly the same genetic code, and biologists now purport to have discerned a Last Universal Common Ancestor (LUCA, for short) - “a tiny, single celled organism, no more than a lipid membrane that encase(d) an early but functioning genome composed of DNA, as well as proteins and the RNA with which these parts communicate(d)”[1], which into being more than 3.5 billion years ago. In this day and age few reputable scientists would argue against the doctrine of common descent [2], but is the evidence sufficient to support the idea of a single common ancestor?
In their Brief History of Creation, Mesler and Cleaves trace the successive stages of classification involved, beginning with the seminal contribution of the Swedish botanist Carl Linnaeus (1707-1778), the father of modern taxonomy, who originally classified organisms into three distinct “kingdoms” – the plants, the animals and the minerals in his Systema naturae - The System of Nature - in 1735. With continued research and as new species were discovered and further fossils found, the Tree of Life grew and continued to grow, until by the time the tenth edition was published in 1758, Linnaeus’ classifications - based on easily observable physical traits such as whether things moved or grew, swam of flew, and/or had fur or backbones - had grown to include 7,700 species of plants and 4,400 species of animals, all systematically grouped and classified. By the advent of the twenty-first century, these classifications and their various sub-categories had exploded exponentially[3].
As techniques in microbiology improved, organisms were further divided into single-celled and multicellular organisms, and later into two different categories: organisms with a cell nucleus called the eukaryotes and those without: the prokaryotes. Incidentally, these prokaryotes have been described as "a family of impoverished, single-celled bacteria who couldn't even afford nuclei" from whom we all descended.[3A] Eventually, the animal world came to be divided into 5 kingdoms: animals, plants, fungi, single-celled eukaryotes and prokaryotes.
In 1977, some 250 years after Linnaeus original publication, it was time for another seminal contribution, this time at the hands of the American bio-physicist Carl Woese (1928-2012) who propounded the idea, highly controversial at the time, that instead of two ancient lineages – eukaryotes and prokaryotes – there were in fact three, all of which branched off separately soon after the beginning of life. The third member of this exclusive club he dubbed archaebacteria (later shortened to “archaea” - the ancient ones) - single-celled organisms which often survived in extreme environmental conditions.
Woese proceeded to trace the myriad of species to its putative origins from the top down, rather than the traditional bottom up approach. In doing so, he found that the oldest life forms shared their genetic information in a collective genetic pool via a process called “horizontal gene transfer’, which explained the code’s resistance to change over billions of years via myriads of species[4]. By these means, early microbes were able to speed their own evolution by drawing on a much larger gene pool, and with primitive organisms sharing information so freely, the root of the tree of life metamorphosed into something “more like a web, connected not by traditional straight-line patterns of descent but by criss-crossing connections established by horizontal gene transfer”[5].
These ideas had the effect of undermining Darwin’s idea of a single common ancestor from which all modern organisms sprang. Woese thought that, tracing the tree of life back to a single ancestor was impossible, and that the deepest we can see back into that line is “a roiling mess of indiscrete organisms evolving in an independent fashion”[6].
With the aid of the full range of fossil molecular and other evidence now available to them, and assisted by computer programming which enables them to analyse tens of thousands of species at a time, scientists are now attempting to draw a tree of life that includes every known species, which will have about 2 million branches – perhaps more like a bush than a tree, as we shall see. Interestingly enough, animals and plants will take up only a tiny part of the tree, since most biodiversity on Earth is microbial[2].
Richard Dawkins recently described the common ancestor concept in his usual colourful phraseology before the Tree of Life project became a work in progress:
The common ancestor of people and budgerigars lived about 310 million years ago, and the ancestor that the duck-billed platypus shared with us lived 180 million years ago, nearly 3 times as long as the extinction of the dinosaurs. In both instances, the ancestral species split in two, and their two strands went their separate ways for the rest of time. The same ancestral species is shared by all mammals on one side of that early divide, and all reptiles (including birds) on the other[3].
Humans are not descended from monkeys, but we share a common ancestor with them. Nor are we are descended from chimpanzees, but molecular evidence shows that the common ancestor we share with them lived about 6 million years ago. Neither are humans are descended from lungfish, but we share an ancestor with them, and it is generally accepted that bats and humans also share a common ancestor. All birds have inherited their feathers from their shared ancestor, which had feathers, and no mammal is descended from that ancestor[4].
So, every animal is linked to every other animal by a chain of intermediates, let’s say by way of example rabbit to leopard since we are about to consider that relationship in a little more detail, and there are similar bridges from rabbit to wombat, from leopard to lobster, from every animal or plant to every other - including humans, and there is simply no evolutionary justification for the common assumption that evolution is somehow “aimed” at humans, or that humans represent “evolution’s last word”, or that man is higher than the other animals or species. It is nonsense to rank species on a ladder[5].
So a rabbit is not “just a rabbit” ...
Mention was made of the hypothetical link between rabbit and the leopard, via a chain of intermediates. When you look closely at each of those species, bear in mind that what you are now observing is not just a static being to which we give the denomination ‘rabbit’ or ‘leopard’. Each is, and continues to be, an evolving being. Now imagine that you are tracing, say, the rabbit back in time from the one you now see to its parent to grandparent to great-grand-parent, way way back in time until the rabbits just start to look a little different to the species we know today until it metamorphoses into an intermediate, let’s say something like a shrew. Then turn an evolutionary hairpin bend and move forward in time. For a while you will be following this shrew like creature until gradually it manifests itself into something more and more like a leopard and so on until we reach the species we know as a leopard today. All along the route on both sides there are numerous other offshoots we could have followed as different rabbit and leopard like species metamorphose into and from each other. We could just as easily have followed separate routes from porcupine to dolphin, from wallaby to giraffe, or from human to haddock[6].
That all sounds very interesting – even exotic perhaps - but there is a practical aspect to this with significant implications for modern medicine, which uses and experiments upon model systems to study more complicated ones, relying on the relatedness of all biological organisms, ourselves included. As Stuart Firestein points out: “It is the process of evolution, the mechanics of genetic inheritance and occasional mutation, that have conserved the genes responsible for making the proteins that confer electrical activity on neurons, as well as those that make kidneys and livers and hearts and lungs work the way they do. If that were not the case, then we couldn’t study these things in worms, flies, rats, mice or monkeys, and believe that it would have relevance to humans. There would be no drugs, no surgical procedures, no treatments and no diagnostic tests. All of these have developed using model systems ranging from cells in culture dishes to rodents to primates. No evolution, no model systems, no progress”[7]. Firestein himself was the recipient of a grant to study the olfactory system of the salamander, not because he had any particular fascinations with the salamanders’ sense of smell, but because they are an excellent model system for working out how brains direct molecules and how new brain cells might be generated and how smell plays a role on sex and production[8].
[1] Darwin, Origin of Species, Penguin, 2009, Chapter on the imperfections of the geological record, at 251.
[2] Carl Zimmer, “Scientists branch out to find missing links in the tree of life”, New York Times, reproduced in the Health and Science segment of the Sydney Morning Herald, 7 June 2002.
[3] The closest relatives of birds (whose family includes dinosaurs and snakes and reptiles) are to be found among the long extinct dinosaurs: Greatest Show, 160..
[4] The sources for this paragraph appear in Greatest Show at 425,152,153, 187,188, 164, 312 and 298
[5] Sources for paragraph: Greatest Show, 23-24, 158.
[6] This is an abbreviated form of the scenario depicted in Greatest Show at 23-25.
[7] Stuart Firestein, Ignorance – How it drives Science, OUP, New York, 2012, 72-73.
[8] Ibid, 75-76.
Is there in fact a “Last Universal Common Ancestor” (LUCA)?
It was Darwin who originally postulated the idea of a single common ancestor – “one primordial form” - from which all modern beings must have descended. In the fullness of time Darwin’s notion of universal common ancestry came to be supported by the fact that every organism, from microbes to human beings, uses almost exactly the same genetic code, and biologists now purport to have discerned a Last Universal Common Ancestor (LUCA, for short) - “a tiny, single celled organism, no more than a lipid membrane that encase(d) an early but functioning genome composed of DNA, as well as proteins and the RNA with which these parts communicate(d)”[1], which into being more than 3.5 billion years ago. In this day and age few reputable scientists would argue against the doctrine of common descent [2], but is the evidence sufficient to support the idea of a single common ancestor?
In their Brief History of Creation, Mesler and Cleaves trace the successive stages of classification involved, beginning with the seminal contribution of the Swedish botanist Carl Linnaeus (1707-1778), the father of modern taxonomy, who originally classified organisms into three distinct “kingdoms” – the plants, the animals and the minerals in his Systema naturae - The System of Nature - in 1735. With continued research and as new species were discovered and further fossils found, the Tree of Life grew and continued to grow, until by the time the tenth edition was published in 1758, Linnaeus’ classifications - based on easily observable physical traits such as whether things moved or grew, swam of flew, and/or had fur or backbones - had grown to include 7,700 species of plants and 4,400 species of animals, all systematically grouped and classified. By the advent of the twenty-first century, these classifications and their various sub-categories had exploded exponentially[3].
As techniques in microbiology improved, organisms were further divided into single-celled and multicellular organisms, and later into two different categories: organisms with a cell nucleus called the eukaryotes and those without: the prokaryotes. Incidentally, these prokaryotes have been described as "a family of impoverished, single-celled bacteria who couldn't even afford nuclei" from whom we all descended.[3A] Eventually, the animal world came to be divided into 5 kingdoms: animals, plants, fungi, single-celled eukaryotes and prokaryotes.
In 1977, some 250 years after Linnaeus original publication, it was time for another seminal contribution, this time at the hands of the American bio-physicist Carl Woese (1928-2012) who propounded the idea, highly controversial at the time, that instead of two ancient lineages – eukaryotes and prokaryotes – there were in fact three, all of which branched off separately soon after the beginning of life. The third member of this exclusive club he dubbed archaebacteria (later shortened to “archaea” - the ancient ones) - single-celled organisms which often survived in extreme environmental conditions.
Woese proceeded to trace the myriad of species to its putative origins from the top down, rather than the traditional bottom up approach. In doing so, he found that the oldest life forms shared their genetic information in a collective genetic pool via a process called “horizontal gene transfer’, which explained the code’s resistance to change over billions of years via myriads of species[4]. By these means, early microbes were able to speed their own evolution by drawing on a much larger gene pool, and with primitive organisms sharing information so freely, the root of the tree of life metamorphosed into something “more like a web, connected not by traditional straight-line patterns of descent but by criss-crossing connections established by horizontal gene transfer”[5].
These ideas had the effect of undermining Darwin’s idea of a single common ancestor from which all modern organisms sprang. Woese thought that, tracing the tree of life back to a single ancestor was impossible, and that the deepest we can see back into that line is “a roiling mess of indiscrete organisms evolving in an independent fashion”[6].
Meet the Concestors [7]
What about common ancestors along the way. This aspect is comprehensively dealt with in a hard copy text entitled The Ancestor’s Tale – A Pilgrimage to the Dawn of Life, Richard Dawkins and Yan Wong, Weidenfeld and Nicolson (2004), 2nd Edition, 2016, and even more alluringly in their interactive webpage http://www.ancestorstale.net/ which reduces the whole of life on earth to a single zoomable page starting from the present (homo sapiens) then working backwards to the earliest bacterial life forms per medium of some 40 steps or “rendezvous” points. [8]
In order to avoid what they term the ‘conceit of hindsight’ (because biological evolution has no privileged line of descent and no designated end), Dawkins and Wong proceed backwards from present day human kind - homo sapiens, in other words - to the bacterial beginnings of life on earth, periodically pausing when one species rendezvous with another with whom they have a common ancestor. “Evolution has reached many millions of interim ends (the number of surviving species at the time of observation), and there is no reason other than vanity (human vanity) to designate any one as more privileged or climactic than the other".
Why a backwards chronology? “In a backward chronology, the ancestors of any set of species must eventually meet at a particular geological moment. Their point of rendezvous is the last common ancestor that they all share, (w)hat I shall call their ‘Concestor’: the focal rodent or the focal mammal or the focal vertebrate, say. The oldest concestor is the grand ancestor of all surviving life”.
Dawkins and Wong depict this backwards journey as a pilgrimage, something like that depicted in Chaucer’s Canterbury Tales with each species - animals, plants, fungi, archaea - commencing from their own individual starting points on their own separate pilgrimages to visit their own ancestors, including the ones they share with us. However, unlike most of Chaucer’s pilgrims, Dawkins and Wong’s do not all set out together, although they do set off at the same time – the present - and journey to the past. All these roads lead to the origin of life, the authors' metaphorical Canterbury, but because we are human, the past we follow is that of our own ancestors: a human pilgrimage to discover human ancestors, along the way meeting and greeting other pilgrims who join us in strict order as we reach the common ancestors we share with them.
The General Prologue [9]
The Canterbury Tales commenced with a General Prologue and so do our learned authors, incorporating a number of “aids to our vision which will help us peer into theatres of ancient vision and reconstruct the scenes and the players, their exist and their entrances, of long ago” – aids such as hard relics or fossils (the oldest bacterial fossils found so far date to about 3.5 billion years ago, so one may infer that the origin of life must at least be earlier than that), then what the learned authors describe as “renewed relics” (secondary evidence such as eye-witness documentary accounts recorded and passed down through the generations, and indeed DNA), and thirdly “triangulation” – the technique of comparing the external characteristics/proteins/DNA sequences of two or more present day organisms/animals/species in order to arrive at a fair and plausible idea of what their ancestors would have looked like.
We all tend to have somewhat of a fascination with fossils as the pre-eminent investigative tool in this area, but as Dawkins and Wong opine: "(i)n spite of the fascination of fossils, it is surprising how much we would still know about our evolutionary past without them. If every fossil were magicked away, the comparative study of modern organisms, of how their patterns of resemblances, especially of their genetic sequences, are distributed among species, and of how species are distributed among continents and islands, would still demonstrate, beyond all sane doubt, that our history is evolutionary, and that all living creatures are cousins. Fossils are a bonus …. (but without them) the evidence for evolution would still be overwhelmingly strong. At the same time, if we had only fossils and no other evidence, the fact of evolution would again be overwhelmingly supported. As things stand at the moment, we are blessed with both". And they also recount elsewhere in the same text: "Even if we had no fossils, a sophisticated comparison of modern animals would permit a plausible reconstruction of their ancestors". [10]
Proceeding from prologue to journey, Concestor 0 is depicted as a special case, being the most recent ancestor of all surviving humans and ranging from Homo Sapiens back to ape-men such as ardipithecus ramidus. Then we cross paths initially with chimpanzee/bonobos (rendezvous 1), then gorillas, orangutans, old world monkeys and new world monkeys, then various other groupings of mammals and so forth “until eventually all the pilgrims of life are marching together in one single backward quest for the origin of life itself”, passing, surprisingly enough, only a little over 40 rendezvous points in all”, and the distant target which is our goal: the grand ancestor of all life.
Concestor 2 whom we meet at rendezvous 2, “is the most recent common ancestor of gorillas on the one hand and {{humans + {chimpanzees + bonobos}} on the other, and Concestor 3 is the most recent common ancestor of orangutans and {{humans + {chimpanzees +bonobos}} + gorillas}. The final Concestor is the grand ancestor of all surviving life forms.
We cannot obviously cover all the concestors and rendezvous in a tome of such magnitude, but I do wish to pay special attention to Rendezvous 35 [11], well back in our march back towards the dawn of life, where the ancestors we meet are the fungi, the second of the great multicellular kingdoms, the third being the plants. In fact the fungi are more closely related to animals than to plants, and feed by means of “a subterranean network of threads called hyphae”, the collection of hyphae belonging to one individual fungus being called the mycelium. Dawkins and Yeo instance a particular fungus called the phallus impudicus (common name "elegant stinkhorn"a photograph of which appears in plate 48 and is entitled "an orgy of mushrooms", which spreads its mycelium over millions of square centimetres, secreting digestive enzymes and digesting the soil material where it lies. I found this of special interest because a representative of this ancient order with origins stemming back to the early stages of the dawn of life on this planet recently visited my garden in suburban Sydney, NSW, Australia, as the accompanying image will attest.
Whether all known life forms, and in particular us, are in fact capable of being traced to a single ancestor which lived more than 3.8 billion years ago, the fact is that this grand confluence of all surviving life is not coincident with the origin of life itself in any event, the very issue which is considered on the next page.
What about common ancestors along the way. This aspect is comprehensively dealt with in a hard copy text entitled The Ancestor’s Tale – A Pilgrimage to the Dawn of Life, Richard Dawkins and Yan Wong, Weidenfeld and Nicolson (2004), 2nd Edition, 2016, and even more alluringly in their interactive webpage http://www.ancestorstale.net/ which reduces the whole of life on earth to a single zoomable page starting from the present (homo sapiens) then working backwards to the earliest bacterial life forms per medium of some 40 steps or “rendezvous” points. [8]
In order to avoid what they term the ‘conceit of hindsight’ (because biological evolution has no privileged line of descent and no designated end), Dawkins and Wong proceed backwards from present day human kind - homo sapiens, in other words - to the bacterial beginnings of life on earth, periodically pausing when one species rendezvous with another with whom they have a common ancestor. “Evolution has reached many millions of interim ends (the number of surviving species at the time of observation), and there is no reason other than vanity (human vanity) to designate any one as more privileged or climactic than the other".
Why a backwards chronology? “In a backward chronology, the ancestors of any set of species must eventually meet at a particular geological moment. Their point of rendezvous is the last common ancestor that they all share, (w)hat I shall call their ‘Concestor’: the focal rodent or the focal mammal or the focal vertebrate, say. The oldest concestor is the grand ancestor of all surviving life”.
Dawkins and Wong depict this backwards journey as a pilgrimage, something like that depicted in Chaucer’s Canterbury Tales with each species - animals, plants, fungi, archaea - commencing from their own individual starting points on their own separate pilgrimages to visit their own ancestors, including the ones they share with us. However, unlike most of Chaucer’s pilgrims, Dawkins and Wong’s do not all set out together, although they do set off at the same time – the present - and journey to the past. All these roads lead to the origin of life, the authors' metaphorical Canterbury, but because we are human, the past we follow is that of our own ancestors: a human pilgrimage to discover human ancestors, along the way meeting and greeting other pilgrims who join us in strict order as we reach the common ancestors we share with them.
The General Prologue [9]
The Canterbury Tales commenced with a General Prologue and so do our learned authors, incorporating a number of “aids to our vision which will help us peer into theatres of ancient vision and reconstruct the scenes and the players, their exist and their entrances, of long ago” – aids such as hard relics or fossils (the oldest bacterial fossils found so far date to about 3.5 billion years ago, so one may infer that the origin of life must at least be earlier than that), then what the learned authors describe as “renewed relics” (secondary evidence such as eye-witness documentary accounts recorded and passed down through the generations, and indeed DNA), and thirdly “triangulation” – the technique of comparing the external characteristics/proteins/DNA sequences of two or more present day organisms/animals/species in order to arrive at a fair and plausible idea of what their ancestors would have looked like.
We all tend to have somewhat of a fascination with fossils as the pre-eminent investigative tool in this area, but as Dawkins and Wong opine: "(i)n spite of the fascination of fossils, it is surprising how much we would still know about our evolutionary past without them. If every fossil were magicked away, the comparative study of modern organisms, of how their patterns of resemblances, especially of their genetic sequences, are distributed among species, and of how species are distributed among continents and islands, would still demonstrate, beyond all sane doubt, that our history is evolutionary, and that all living creatures are cousins. Fossils are a bonus …. (but without them) the evidence for evolution would still be overwhelmingly strong. At the same time, if we had only fossils and no other evidence, the fact of evolution would again be overwhelmingly supported. As things stand at the moment, we are blessed with both". And they also recount elsewhere in the same text: "Even if we had no fossils, a sophisticated comparison of modern animals would permit a plausible reconstruction of their ancestors". [10]
Proceeding from prologue to journey, Concestor 0 is depicted as a special case, being the most recent ancestor of all surviving humans and ranging from Homo Sapiens back to ape-men such as ardipithecus ramidus. Then we cross paths initially with chimpanzee/bonobos (rendezvous 1), then gorillas, orangutans, old world monkeys and new world monkeys, then various other groupings of mammals and so forth “until eventually all the pilgrims of life are marching together in one single backward quest for the origin of life itself”, passing, surprisingly enough, only a little over 40 rendezvous points in all”, and the distant target which is our goal: the grand ancestor of all life.
Concestor 2 whom we meet at rendezvous 2, “is the most recent common ancestor of gorillas on the one hand and {{humans + {chimpanzees + bonobos}} on the other, and Concestor 3 is the most recent common ancestor of orangutans and {{humans + {chimpanzees +bonobos}} + gorillas}. The final Concestor is the grand ancestor of all surviving life forms.
We cannot obviously cover all the concestors and rendezvous in a tome of such magnitude, but I do wish to pay special attention to Rendezvous 35 [11], well back in our march back towards the dawn of life, where the ancestors we meet are the fungi, the second of the great multicellular kingdoms, the third being the plants. In fact the fungi are more closely related to animals than to plants, and feed by means of “a subterranean network of threads called hyphae”, the collection of hyphae belonging to one individual fungus being called the mycelium. Dawkins and Yeo instance a particular fungus called the phallus impudicus (common name "elegant stinkhorn"a photograph of which appears in plate 48 and is entitled "an orgy of mushrooms", which spreads its mycelium over millions of square centimetres, secreting digestive enzymes and digesting the soil material where it lies. I found this of special interest because a representative of this ancient order with origins stemming back to the early stages of the dawn of life on this planet recently visited my garden in suburban Sydney, NSW, Australia, as the accompanying image will attest.
Whether all known life forms, and in particular us, are in fact capable of being traced to a single ancestor which lived more than 3.8 billion years ago, the fact is that this grand confluence of all surviving life is not coincident with the origin of life itself in any event, the very issue which is considered on the next page.
[1] Bill Mesler and James Cleaves III, A Brief History of Creation – Science and the Search for Life, W.W. Norton & Co, New York, 2016, pp 223-4.
[2] Ibid 228-9.
[3] As revealed in some detail in ibid, 227-8.
[3A] Danny Katz, "Modern guru", Sydney Morning Herald, Good Weekend, 25 February 2017 - a description so evocative that I couldn't resist citing it again. It also appears at The evolutionary garden
[4] Ibid, 233-4.
[5] Ibid, 236.
[6] Ibid.
[7] Dawkins and Wong, 1-11.
[8] For a graphic depiction of the tree of life in all its domains purporting to stem from LUCA see https://www.bighistoryproject.com/chapters/3#tree-of-life and as regards the role of humanity: Humans - Out of Africa - /humans---out-of-africa.html See also http://opentreeoflife.org/ and "The Circle of Life - Lineage of all known species on earth are being pieced together", Scientific American, March 2016, 68.
[9] Dawkins and Wong, 12-26.
[10] Ibid, 13; and 24.
[11] Ibid 563, 565.