Skip to comments.Evolution: hacking back the tree of life (can anyone say DEVOLUTION?)
Posted on 11/14/2007 4:00:52 PM PST by GodGunsGuts
Evolution: hacking back the tree of life
13 June 2007
NewScientist.com news service
If you want to know how all living things are related, don't bother looking in any textbook that's more than a few years old. Chances are that the tree of life you find there will be wrong. Since they began delving into DNA, biologists have been finding that organisms with features that look alike are often not as closely related as they had thought. These are turbulent times in the world of phylogeny, yet there has been one rule that evolutionary biologists felt they could cling to: the amount of complexity in the living world has always been on the increase. Now even that is in doubt.
While nobody disagrees that there has been a general trend towards complexity - humans are indisputably more complicated than amoebas - recent findings suggest that some of our very early ancestors were far more sophisticated than we have given them credit for. If so, then much of that precocious complexity has been lost by subsequent generations as they evolved into new species. "The whole concept of a gradualist tree, with one thing branching off after another and the last to branch off, the vertebrates, being the most complex, is wrong," says Detlev Arendt, an evolutionary and developmental biologist at the European Molecular Biology Laboratory in Heidelberg, Germany.
The idea of loss in evolution is not new. We know that snakes lost their legs, as did whales, and that our own ancestors lost body hair. However, the latest evidence suggests that the extent of loss might have been seriously underestimated. Some evolutionary biologists now suggest that loss - at every level, from genes and types of cells to whole anatomical features and life stages - is the key to understanding evolution and the relatedness of living things. Proponents of this idea argue that classical phylogeny has been built on rotten foundations, and tinkering with it will not put it right. Instead, they say, we need to rethink the process of evolution itself.
It is not hard to see how the mistake might have happened. In the past, the tree of life was constructed on the basis of similarity of morphological features. The more similar two species looked, the more closely related they were thought to be. But looks can be deceptive. This became abundantly clear more than a decade ago, when molecular biologists began comparing small numbers of genes from various organisms and found that many species were not what they appeared. Hippos, for example, were once thought to be the kissing cousins of pigs, but genetic evidence revealed their closest living relatives to be the cetaceans (whales, dolphins and porpoises).
Without the insights of molecular analysis, traditional morphologists also had no way of knowing whether a particular feature had been lost in a given lineage, or had never been there in the first place. In line with the idea that things evolve towards increasing complexity, they tended to assume the latter, sometimes quite incorrectly. Take the sea squirt. Its larva swims around looking like a tadpole, with a nerve cord along its back, gill slits for feeding and a tail - all classic features of chordates, the large group of animals with backbones that includes us. Then, however, it stands on its head and turns into a sack of jelly, having first digested what it had of a brain. The adult looks suspiciously like a plant. For a long time it was considered to be one of the most primitive chordates because of its simple adult form - about as far from vertebrates as it was possible to get. In between were myriad other groups, including the lancelets - fish-like animals that hang on to their nerve cords into adulthood. Then molecular studies revealed that sea squirts are genetically closer to us than are lancelets, and the tree had to be reshuffled.
In recent years, genetic analysis has forced biologists to consider the possibility that organisms such as the sea squirt might have lost some of the complexity of their ancestors. Yet even now, few recognise the full implications of loss as a key player in evolution. The entire tree of life has been built on the assumption that evolution entails increasing complexity. So, for example, if two groups of animals were considered close because both had a particular prominent feature, then someone discovered a third, intermediate line that lacked that feature but shared many other aspects of the two groups, traditional phylogenists would conclude that the feature had arisen independently in the two outlying groups, by a process known as convergent evolution. They often did not even consider the alternative explanation: that the feature in question had evolved just once in an ancestor of all three groups, and had subsequently been lost in the intermediate one. Now a handful of molecular biologists are considering that possibility.
Instead of simply looking to see whether two species share certain genes, the new approach involves taking the "molecular fingerprint" of different types of cells. It identifies the unique combination of transcription factors - molecules that control which of a cell's genes are turned on and when - that specify the make-up of a cell, including the molecular signals it transmits and receives. If two groups of organisms share the same type of cells, with the same molecular fingerprint, giving rise to similar features in both, then it is extremely unlikely that these features evolved twice. So any intermediate groups of organisms that lack that feature would most likely have lost it during the course of evolution. Only now, with the ability to explore at the molecular level how morphological features have been lost, gained and modified over time, is the true extent of evolutionary loss coming to light.
Arendt's convictions about the vast scale of this loss are based on his molecular fingerprinting studies of a tiny annelid worm called Platynereis dumerilii. It is an unprepossessing animal that lives in tubes stuck to rocks in shallow seas, bathed in a nutritious blanket of algae and reproducing according to the tides and the lunar cycle. "We think that it has always lived in this ecological niche," he says, "and that this might resemble the environment of the common ancestor [of all animals that are symmetrical along the axis from head to tail]". This enormous group, called bilaterians, encompasses all vertebrates and most invertebrates; the descendants of a long-extinct creature known as Urbilateria that lived between half a billion and a billion years ago. No fossils of this species have been found, but as Platynereis is thought to have occupied the same niche as Urbilateria, Arendt suspects it might also have retained some of the mysterious ancestor's features.
Brainy ancestors What Arendt's group has discovered about the brain of this lowly worm is intriguing. Within the animal kingdom, the simplest and most evolutionarily ancient type of nervous system is a diffuse neural net. Sea anemones and corals, for example, have this system, in which a single type of neuron is distributed throughout the animal. More recently evolved species have a central nervous system (CNS), with specialised sensory and motor neurons clumped together into a nerve cord and brain.
A CNS is found in all vertebrates and some invertebrates, including Platynereis and two of biology's supermodels, the fruit fly Drosophila melanogaster and the nematode worm Caenorhabditis elegans, but there are obvious differences between the vertebrate and invertebrate CNS. Vertebrates have a spinal cord at the back, while invertebrates usually have a chain of neuronal clusters or ganglia, connected like a rope ladder, in their belly. This led morphologists to think that Urbilateria had a diffuse neural net and that centralisation arose separately by convergent evolution in the different lines after they split. Arendt believes they were wrong.
Earlier this year, his group reported that Platynereis neurons share molecular fingerprints with vertebrate neurons during development (Cell, vol 129, p 277). For example, genes known to be important in patterning the vertebrate CNS also divide the worm's nervous system into domains. What's more, domains with corresponding gene expression patterns give rise to the same types of neurons in both. Arendt concludes that Platynereis and vertebrates both inherited their CNS from Urbilateria. The reason they take a different form today, he suggests, is that when early vertebrates began swimming freely, "front" and "back" lost their significance and the animals simply inverted the two. As the rope ladder nervous system became enclosed in the neural tube characteristic of vertebrates, the ancestral mouth was trapped inside. It is still detectable there, Arendt says. Using the molecular fingerprinting technique he has been able to find this obsolete mouth within the vertebrate brain. "Its position is very clear," he says, "It's behind the hypothalamus."
If Arendt is correct, then the ancestral CNS was lost completely in two major animal groups: the echinoderms (starfish, sea urchins and the like) and hemichordates (acorn worms and other worm-like marine organisms). Both of these are sister phyla to chordates (see Diagram), yet their members have lost their brains and instead have diffuse neural nets. The same seems to be true of various molluscs, brachiopods, phoronids and bryozoans that have evolved to be sedentary filter-feeders. "If you just sit around your entire life you don't need much of a sensory integration centre coupled to a locomotor nerve cord," says Arendt.
Not everyone agrees, however. In 2003, Chris Lowe from the University of Chicago and colleagues compared the genes expressed in the development of the acorn worm and vertebrate nervous systems. "We showed that the exact same genes are involved in patterning a nerve net as in patterning a CNS," he says. "So our argument is that you cannot use these genes as really solid markers of a CNS." Given the scarcity of comparative molecular data so far, Lowe thinks it is too early to rule out convergent evolution in annelids and vertebrates.
While controversy continues to rage over convergent evolution versus loss, it has emerged that Urbilateria is not the only very early animal ancestor that was more complex than some of its descendants. David Miller of James Cook University in Queensland, Australia, studies the coral genus Acropora, the main reef-building corals of the Indo-Pacific region. Acropora belongs to the phylum of cnidarians, which are thought to have branched off after Urmetazoa, the common ancestor of all animals, but before Urbilateria. Yet Miller is uncovering surprising genetic complexity in Acropora.
For example, it has a version of a gene that was thought to be exclusive to vertebrates as it is involved in the vertebrate immune system, which works by remembering past threats and adapting its response to them. "All the textbooks tell you that adaptive immunity is a specific characteristic of vertebrates," Miller says, "yet at least one of the proteins is clearly present in our animal."
Miller's findings are intriguing, but more work is needed to pin down the origins of adaptive immunity. The dangers of jumping to conclusions about early evolution followed by loss on the basis of limited genetic information are highlighted by work on body segmentation. Many bilaterians have bodies made up of repeating anatomical units, and the discovery of certain similarities between the developmental genes that determine segmentation in Platynereis and in insects suggested a common origin. Then Elaine Seaver at the University of Hawaii's Kewalo Marine Lab in Honolulu used molecular fingerprinting to show that almost none of the genes involved in insect or vertebrate segmentation are deployed in the same place at the same time in developing annelids. "The evidence is accumulating that segmentation arose at least three times independently," says Seaver's colleague Mark Martindale.
Now that the spectre of loss has been raised, however, proponents of the new model see it everywhere - everywhere, at least, where animals have evolved to occupy niches in which their pre-existing complexity might be superfluous. Last month, Marcus Davis of the University of Chicago and colleagues reported that a species of paddlefish shows patterns of gene expression during development that were previously thought to be exclusive to land-living vertebrates - in other words, those with limbs. This paddlefish is the living species that most closely resembles the bony fish of the Palaeozoic era, which lived more than 250 million years ago. Davis concludes that primitive bony fish may have had something like limbs, which were lost in their descendants (Nature, vol 447, p 473).
Parasitism is another potential driver of simplification. When one living organism colonises another, it may discard features it could not have survived without as a free-living creature - features that gave it mobility and the ability to seek out food, for instance. Max Telford from University College London gives the example of a genus of barnacle called Sacculina. Barnacles are crustaceans that don't look much like crustaceans because they are filter-feeders and sessile, meaning they anchor themselves to a substrate - often boats or piers. Sacculina doesn't look like a barnacle, let alone a crustacean. It parasitises crabs, producing an almost plant-like system of roots which invades the host tissue. It is known to be a barnacle only because it has a barnacle-like larval stage. "So barnacles have lost many crustacean characters because they are sessile, and Sacculina has gone even further because it is parasitic," says Telford.
Another driver of simplification might be miniaturisation. Rotifers are microscopic aquatic organisms with a feeding wheel - tufts of cilia around the mouth that waft food into it - and nothing that could strictly be called a brain. Arendt believes that they represent the larval stage of an animal that, on shrinking to fit its planktonic niche, discarded its adult body plan and developed no further, becoming sexually mature early. "This is one very efficient means of throwing out ancestral complexity and becoming secondarily simple, and I think it happens frequently," he says.
If loss is so common, the challenge now is to distinguish the organisms that were always simple from those that have evolved simplicity. Genetics will be an invaluable tool here, but it will take a lot more analysis and comparison between a wide range of species before a definitive tree of life emerges. The very genetic complexity of Acropora, for example, has led some to question its position in the tree, arguing that it may have evolved later than was thought - that it may in fact be a descendant of Urbilateria that became secondarily simple while retaining genes that were later incorporated into the vertebrate immune system. If evolutionary biologists today are to avoid the mistakes of their predecessors, they need to eliminate precisely that kind of circularity.
"Molecular biology is making real inroads into this, but it has not been easy to reconstruct events that happened over half a billion years ago," Martindale says. Still, the new phylogenists are more resolute than ever. "There can only be one true relationship of animals to one another," he says.
Just as I suspected. The evidence in favor of creation is growing with each passing day.
Forty-plus years as a life-scientist and I never heard of this rule. Must be a straw man.
Thanks so much for the ping, GGG! Bookmarked for later perusal....
Take off rose-colored glasses and read for content. This article is not supporting creationism, it is supplying additional details for evolution. In doing so, the theory of evolution is becoming more accurate, not less.
Note also that half billion year figure. That would seem to be a bit hard to reconcile with YEC, but I don't doubt you will manage somehow.
Nope. The Evolutionists are being forced to come around to what Creationists have been saying all along. The dominant pattern in nature is Devolution, not Evolution.
==This article is not supporting creationism, it is supplying additional details for evolution. In doing so, the theory of evolution is becoming more accurate, not less.
You are quite correct. Whenever the theory of evolution veers towards Creationism it automatically becomes more accurate.
==Note also that half billion year figure. That would seem to be a bit hard to reconcile with YEC, but I don’t doubt you will manage somehow.
The evolutionary time frame will also come tumbling down. And then the TOE will be forced to move even closer to Creation Science. Time, as they say, is on our side d:op
>> Since they began delving into DNA, biologists have been finding that organisms with features that look alike are often not as closely related as they had thought. <<
This is the main reason why I found biology intellectually uninteresting. The similar macro-features => similar micro-features logical leap makes biology seem too ridiculous to be real science.
Not on this planet.
==Not on this planet.
It has already begun. Darwin’s TOE is on the way out. It’s only a matter of time.
They just did.
You are wrong..... read up on the Burgess Shale.
The evolutionary tree is constantly changing. Every new textbook has its own drawing. This is not to say that the evolutionary path of life from the rock bacteria to our exalted selves has changed at all, but our sleuthing out of how it was is updated all the time. Sometimes they even change the classifications altogether as they try to make some kind of coherent sense of the data collected so far. Even just a hundred years ago when Einstein had his big year 1905 the whole game was the one and only galaxy and it was only a billion years old. More data and more all the time and the picture has to change. It’s good or there would be little point to writing a doctoral thesis anymore.
Not so fast. There is no fossil evidence for a single trunk. As Stephen Jay Gould explains, the evidence points in the opposite direction:
The history of most fossil species include two features particularly inconsistent with gradualism:
1) Stasis - most species exhibit no directional change during their tenure on earth. They appear in the fossil record looking much the same as when they disappear; morphological change is usually limited and directionless;
2) Sudden appearance - in any local area, a species does not arise gradually by the steady transformation of its ancestors; it appears all at once and ‘fully formed’.
Gould, S.J. (1977)
“Evolution’s Erratic Pace”
Natural History, vol. 86, May
Devolution is Evolution. They are exactly the same. Why do we have two words? Because we can.
They would remain in their form since they are fully evolved as such. Every species we see on earth now is fully evolved and would not exhibit directional change. If something evolves from that it would be a new species.
Seperate origins is not Darwinian evolution. No where did the article even suggest this to be the case. All they do is suggest that these ancestors are likely to be more complex than their descedents.
Furthermore, no evolutionist, getting paid by our taxes, would ever make a statement like yours. It would be the equivalent of stating "please shut off all our funding, our work is bogus." Not going to happen. The college deans would go nuts over anything that would threaten their revenue stream.
I read the article and didn't come to that conclusion. Could you throw me some evidence for that statement.
Yes. So is this mighty creationist idea. There is another idea that has nothing to do with stochastics.
==Devolution is Evolution. They are exactly the same. Why do we have two words? Because we can.
They are not the same. The TOE would have us believe that life’s pattern is best represented by a single tree that began with lower organisms that evolve into higher organisms. Creationists believe life’s pattern displays just the opposite. Namely, that life began with the creation of the highest organisms, and is slowly devolving into lower organisms. Big difference.
==I read the article and didn’t come to that conclusion. Could you throw me some evidence for that statement.
You do have to have a very fertile imagination to be an evolutionist. And also be most credulous.
The TOE doesn’t require or infer increasing complexity. That is a later addition by exegetes of limited cognizance. Stochastics has nothing to do with it either, but that is a different idea.
Remember the old "missing link" business? What we have here are some "missing classes, orders and families" ~ a lot more than a "link" sort of thing.
None of this is surprising if life, per se, in all of its various locations throughout the universe, shares the same fundamental engines ~ they should be expected to be shuffled a bit differently (but otherwise utilize the same DNA) coming from different places.
There is not a general trend toward complexity, and humans are not more complicated than amoebas. Wherever that 'nobody' statement came from it can go back now.
That would be fine with me, but the purpose of the space program is not actually to search for life or even to do science. It has a state purpose.
I think it was the part about the critters losing their brains. That’s rather serious you know.
Once we get SDI out of the way (and working I might add), it’s time to search the other planets for functioning protein-making machinery.
They’ve evolved an efficient way to continue eating and breeding by use of a certain system. This pattern of development is an advance for that species. Humans have an appendix they don’t use, is that ‘devolution’ or a vestigial remains of a time when our species DID have a use for it?
There is some doubt that our CNS including the brain parts are necessary to consciousness. It is an old question. Some creatures exhibit clear responsiveness and behavior with no hint of a nervous system at all, even our cousins the plants do that.
If it’s there we’ll stumble across it. We probably already have. In the meantime space programs are the state equivalent of high-marking contests.
The whole notion was popularized by Darwin himself in his "Orgin of Species." Here's the chart from his book:
And here's his explanation of the "great Tree of Life":
"The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth. The green and budding twigs may represent existing species; and those produced during each former year may represent the long succession of extinct species. At each period of growth all the growing twigs have tried to branch out on all sides, and to overtop and kill the surrounding twigs and branches, in the same manner as species and groups of species have tried to overmaster other species in the great battle for life. The limbs divided into great branches, and these into lesser and lesser branches, were themselves once, when the tree was small, budding twigs; and this connexion of the former and present buds by ramifying branches may well represent the classification of all extinct and living species in groups subordinate to groups. Of the many twigs which flourished when the tree was a mere bush, only two or three, now grown into great branches, yet survive and bear all the other branches; so with the species which lived during long-past geological periods, very few now have living and modified descendants. From the first growth of the tree, many a limb and branch has decayed and dropped off; and these lost branches of various sizes may represent those whole orders, families, and genera which have now no living representatives, and which are known to us only from having been found in a fossil state. As we here and there see a thin straggling branch springing from a fork low down in a tree, and which by some chance has been favoured and is still alive on its summit, so we occasionally see an animal like the Ornithorhynchus or Lepidosiren, which in some small degree connects by its affinities two large branches of life, and which has apparently been saved from fatal competition by having inhabited a protected station. As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications."
Devolution is only, and always, a loss of 'part of the blueprint.' If the wind blows away the plan pages for the elevator, and we build the building without it, thus relying on the stairway, it is not an evolution of the art of building.
At the moment we don't know where the seat of consciousness resides, but I'd bet it's in at least one cell located near the main signal switching unit in the brain.
Now a thought like that is a tad heretical since it denies primacy to the brain ~ on the other hand it allows for consideration of all cellular life as repositories for fully conscious (if not knowledgeable) entitites ~
Nikko Tinbergen suggested that 100% of all the known functions of a human being can be found in the single celled paramecium. Lorenz, his contemporary, also discussed King Solomon's Ring, a kind of an icebreaker in any conversation about freewill, consciousness, and just exactly why is that parrot trying to make time with my girl.
Think of multicellular life as a prosthesis for a horribly handicapped, but fantastically intelligent, microscopic lifeform.
1) Stasis - most species exhibit no directional change during their tenure on earth. They appear in the fossil record looking much the same as when they disappear; morphological change is usually limited and directionless;
2) Sudden appearance - in any local area, a species does not arise gradually by the steady transformation of its ancestors; it appears all at once and fully formed.
When did God give us the English Bulldog?
When did God give us the French Poodle?
When did God give us the Boston Terrier?
Because the mathematics of abiogenesis never work. Thus we’ll never see the suggestion that there are multple starting life forms when no one can prove the first one could even come into existence.
You cannot spontanously create the simpliest life form presently known to man without completely defying the laws of probability. This is why many evolutionists will state that there were many, many intermediate lifeforms that evolved between the first life form that came into existance and the single celled amoeba we find today. Those lifeforms are either extinct, or as of yet, undetected.
Continuing, these life forms were much simplier and gained complexity and functions over many successive generations until they become like the single celled amoeba we find today. It really comes down to this, God did it, or space aliens. And if you want to go with space aliens, where did they come from originally?
It’s like the arrow of time. That is a figment, too.