Posts Tagged ‘missing link’

Fossil of Ida (Darwinius masillae)

Fossil of Ida (Darwinius masillae)

She’s back in the news, but just barely. That 47 million-years-old skeleton of what may be the earliest-known primate, Darwinius masillae (aka “Ida”), is getting some more attention — at least, in scientific circles.

As you may recall (go here, then come back), there was a lot of hype last year when Ida was revealed to the world with her being trumpeted as “clear proof of Darwin’s theory of evolution” and the “eighth wonder of the world”. Professor Jorn Hurum et al. were pushing the idea that Ida was not lemur-like enough to be a true strepsirrhine, so she had to be a proto-haplorhine, and therefore a distant “missing link” cousin of modern humans. But, in addition to turning up their noses at the sensationalism surrounding the find, critics said the claims being made about Ida’s place in the “family tree” (or, more accurately, “bush”) were questionable, at best, and the remains required further examination by additional experts.

Now, those experts have completed their study and published their own findings in the Journal of Human Evolution.

Many lines of evidence indicate that Darwinius has nothing at all to do with human evolution,” says Chris Kirk, associate professor of anthropology at The University of Texas at Austin. “Every year, scientists describe new fossils that contribute to our understanding of primate evolution. What’s amazing about Darwinius is, despite the fact that it’s nearly complete, it tells us very little that we didn’t already know from fossils of closely related species.”

As Blythe Williams, lead researcher and anthropologist at Duke University, pointed out:

There’s this enormous body of literature that has built up over the years. The Darwinius research completely ignored that body of literature.”

Those who determined last year that Ida was a haplorhine highlighted certain features she shared with monkeys, apes and humans — namely, a short snout and a deep jaw. But, Williams’ team points out that this is not uncommon, even among strepsirrhines (i.e., lemurs & lorises). In fact, Ida is missing most of the key anatomical features — e.g., a middle ear with two chambers and a plate of bone that shields the eyes from the chewing muscles — that would place her firmly in the haplorhini Suborder. Thus, says Kirk. “[Y]ou can forget about Darwinius being a close relative of humans or other anthropoids.”

I knew it all along….

The 2004 discovery (reported in 2006) of a well-preserved fossil creature dubbed Tiktaalik was hailed as the “missing link” that finally solidified the “fish to tetrapod” transition — a “snapshot” of “a fossil fish in the act of adapting toward a life on land”. Dated to 375-383 Mya, Tiktaalik was concluded to be an intermediary between the sarcopterygian fishes (i.e., Eusthenopteron and Panderichthys) and early tetrapods (i.e., Acanthostega and Ichthyostega). Or, as some call it, a “fishapod”. Some paleontologists have made highly controversial claims of evidence for a “wrist” and fingers, as well as muscular forefins. It had a spiracle, and evidence suggests lungs & gills and eventual development of a more robust ribcage. Along with certain intercranial structures, all these developments were believed to allow Tiktaalik to breathe and support itself on solid ground. Tiktaalik is also the first fish (or, something like it) known to have a “neck”, because it lacked the bony plates in the gill area which restrict head movement.

Some have argued that it is more appropriate to classify Tiktaalik as simply a fish, rather than part-fish/part-amphibian. Notably, its fin is completely finlike, showing no real evidence of transforming into a foot, which is precisely the key feature needed to justify calling it a “transitional form” from fish to tetrapod.  It is also an “inconvenience” that Tiktaalik had no precursors to fingers like Panderichthys did, despite the fact that Tiktaalik is supposed to be further along the evolutionary ladder. Although the Tiktaalik specimens are incredibly well-preserved, they shed little light on the evolution of the soft anatomy requisite for living on land. Furthermore, a true intermediate form would have needed to simultaneously be evolving many features & abilities just to survive its transition from water-dweller to land-dweller — e.g., new ways to keep from drying out on land, specialized structures for breathing oxygen, new methods of obtaining food and water, etc.

In 2008, Jennifer Clack, Per Ahlberg, et al. published their findings on Ventastega, another fossil discovery dating to 365 Mya. This basil tetrapod, which some factors suggest lived in a tidal sea, was “probably more aquatic than terrestrial,” yet “it was more tetrapod than fish….” They classified it as intermediate between the ‘elpistostegids’ (e.g., Panderichthys & Tiktaalik) and Devonian tetrapods (e.g., Acanthostega & Ichthyostega). As far as I can tell, though, they have no actual Ventastega fossils dating earlier than Acanthostega, so this transitional status is no more than “educated speculation”. Plus, it isn’t as “advanced” as some other tetrapods, so it seems to be out of place or anachronistic.

Now, here is what blew everything out of the water, so to speak. Recently, at an old quarry in southeastern Poland were found multiple sets of tracks of a tetrapod — actually, from multiple individuals of different sizes — in what looks to be an ancient marine shoreline. According to Per Ahlberg of Sweden’s Uppsala University, a member of the team that found the tracks, “[They are] fossil of footprints that give us the earliest record of how our very distant ancestors moved out of the water and moved on to the land and took their first steps.”

Tetrapod tracks

397 Mya Tetrapod tracks (borrowed from BBC News)

The footprints, some of which show distinct signs of digits and ankles, date much earlier than they “should”; specifically, they are quite solidly placed at 395-397 million years old. That is 12 million years or more before Tiktaalik, which rules out Tiktaalik as transitional between fish and the first tetrapod. In fact, this now pushes the earliest known tetrapod (though we don’t have an actual skeletal fossil to name, yet) back by at least 18 million years. Furthermore, these trackways “show that the first tetrapods thrived in the sea, trampling the mud of coral-reef lagoons; this is at odds with the long-held view that river deltas and lakes were the necessary environments for the transition from water to land during vertebrate evolution.”

What to make of all this? It is a reminder of a few things that we often need reminding of. First, the oldest known example of a thing is not always the oldest ever. This is just as true in paleontology as it is in other historical sciences, like archaeology. This is often true for the latest known examples, as well. Second, morphologically transitional forms are not always chronologically transitional, and vice versa. That is, Thing X may have features that look to be a mix of Thing A and Thing B, but dating methods sometimes show that Thing X cannot possibly have been a developmental intermediate between A and B. Thus, any theory that assumed such would be false. (Remember the case of the Archaeopteryx?) This is all the more reason to remember that such theories must be held with reserve, always provisional upon things like further physical specimens for evidence, better and more-refined technologies, and mathematical probabilities of events within known parameters. Also, beware of letting assumptions & biases get in the way of facts.

This brings me to my third observation, which is that, once again, the “proof” for neo-Darwinism isn’t nearly as solid as many of its proponents like to proclaim. Indeed, even before this current discovery, there were many competing ideas of how to best classify the various fishes, potential fishapods, and primitive amphibians, precisely because the different mosaics of “transitional” features found in different epochs make it far from clearcut. For example, was Panderichthys a fish or a fishapod? Was Acanthostega a fishapod or a true tetrapod? One’s opinion on matters like these determine where one thinks they should go in the “tree of life”, which, of course, is actually more of a bush. Indeed, many different models have been advanced and then abandoned over the years. The trackways in Poland just throw another wrench into the works.

With the above said, this latest find does NOT necessarily disprove evolutionary theory. It merely means that the EARLIEST tetrapods are older than previously thought AND that they are not quite where they were expected. Paleontologists must now look at Early Devonian (416-397 Mya) formations for even EARLIER transitions. They will also need to find EVEN EARLIER lobe-finned or other fishes as candidates for the tetrapods’ progenitors. (Perhaps Coelacanth will get another chance?) And what of Tiktaalik and the other creatures discussed above? It is still possible that they do represent a separate and unrelated, fish-to-tetrapod transition. But, one has to wonder if the Darwinian process can account for all the genetic & systemic changes needed to go from fully aquatic to land-dwelling, especially in the required timeframe of just a few million years.

Of course, this assumes that the fishes & fishapods actually transitioned into anything new. Perhaps they were stable forms in their own right, well-adapted (or, well-designed, if you prefer) for living in their particular environment. After all, they seem to have been fully-functional, lacking any unfinished, half-formed adaptations. They were not mere works-in-progress.

I’ll finish with this quote from Henry Gee, editor of the journal Nature: “A fairly complete picture of tetrapod evolution, built up over the past twenty years, has been replaced by a blank canvas overnight…. It means that the neatly gift-wrapped correlation between stratigraphy and phylogeny, in which elpistostegids represent a transitional form in the swift evolution of tetrapods in the mid-Frasnian [stage of the Devonian Period], is a cruel illusion. If – as the Polish footprints show – tetrapods already existed in the Eifelian [stage], then an enormous evolutionary void has opened beneath our feet.”

At the very least, the discovery in Poland represents somewhat of a paradigm shift. It should be fascinating to see what evidence future discoveries bring to this debate….

Once again, the evidence flies in the face of evolutionary (i.e., neo-Darwinian) theory. Or, does it?

A new discovery in Poland (see here or here) places the oldest tetrapod several million years earlier than the supposed transitional forms that biologists have been touting as proof of fish evolving into land animals. Naturally, this has caused quite a few excited ripples throughout the scientific community, with evolutionists in general insisting that the discovery isn’t really a problem and Darwin-skeptics pointing out that it really is a problem — possibly even a game-changer. But, everyone agrees that this is a significant find which could lead to further, revolutionary discoveries and advancement in knowledge of the era in question.

Before delving into the new find itself, let’s get a little background on the prevalent theory. First, a “tetrapod” is any four-limbed (but, not finned), usually terrestrial, animal with a spine — from salamanders to lizards to buzzards to Joe Sixpack. Evolutionary biology teaches that the first tetrapods evolved from lobe-finned fish, out of brackish and/or fresh water,
in the Devonian Period, which spans 416 to 359.2 million years ago (Mya). Loooong before the dinosaurs ruled the Earth, obviously, since they’re, like, “advanced” tetrapods.

For quite awhile, evolutionary biologists believed that some of these lobe-finned fish started coming up on land to hunt for prey (like today’s mudskippers) or to search for water when their pond started drying up. Those who were able to stay out of the water for longer periods began adapting and eventually developed lungs, legs, etc. In the late 1980s, British paleontologist Jennifer A. Clack discovered the fossil of an Acanthostega, an early “transitional” tetrapod that turned out to be entirely aquatic, thereby forcing a shift in the consensus view of the “fish to tetrapod” transition. That is, it became apparent that the first tetrapods were not land-dwellers, so the reasons for the transition no longer fit. Some have gone with an “escape-from-predator” scenario as the selective pressure for moving to land. Others speculate the limbs were for “negotiating their way through underwater obstacles” and were later “co-opted for terrestrial use.”

The major “players” are/were as follows:

Coelacanth is probably the best-known of the prehistoric, lobe-finned fishes, primarily because several specimens have actually been found alive in modern times (latest in 1998). It first shows up in the fossil record at 410 Mya and until 1938 was believed to have gone extinct roughly 65 Mya. Some of the coelacanth’s unusual features include a three-lobed tailfin; pectoral, pelvic and anal fins on muscular stalks (or lobes) supported by bones; a special electroreceptive device (i.e., a rostral organ) in the front of the skull, thought to have aided with balance and/or in hunting prey. However, while the coelacanth was originally thought to be a “missing link”, it is now understood to not be part of the direct tetrapod lineage. Rather, it is their sister group, the Rhipidistians. These split into lungfishes and Tetrapodomorphs, both of whom are believed to have developed proto-lungs and proto-limbs — in the middle Devonian (397-385 Mya) and late Devonian (385-359 Mya) epochs, respectively.

Lobe-finned fish and early tetrapods

Lobe-finned fish and early tetrapods (borrowed from Wikipedia)

Often featured in textbooks as a “missing link” dating to about 385 Mya, Eusthenopteron was actually a primitive lobe-finned fish. But, it shared certain features with tetrapods — e.g., certain skull rooting bone patterns, internal nostrils, labyrinthodont teeth, a two-part cranium hinged at mid-length along an intracranial joint, and similar patterns of its fin endoskeleton (i.e., there are bones reminiscent of arms & legs). On the other hand, Eusthenopteron appears to have lacked a clear larval stage and metamorphosis, unlike later tetrapods.

Panderichthys, which lived around 380-385 Mya, had a large head like a tetrapod’s and is considered to have many features that demonstrated an early transitional phase between the lobe-finned fish and primitive tetrapods. For example, “its pectoral girdle shows derived characteristics while its pelvic girdle retains ancestral ones,” so it could at least prop itself up on its muscular forelimbs. Research reported on in 2008 discussed a fanlike array of fingerlike extensions “tucked beneath the fin’s skin and bony scales and rays.” Notably, it also had a spiracle (kind of like a whale’s blowhole), which is held by some to have eventually evolved into the stirrup bone of the human middle ear.

Acanthostega dates to about 365 Mya. It was salamander-like in appearance, with four legs & feet and eight webbed digits on the end of each, but it had no wrists (ankles?) to support itself out of the water. It had primitive lungs, but its short ribs could not yet support its chest cavity on land. Its gills were more like a fish’s than like a modern amphibian’s. Research on Acanthostega‘s skull indicates that it took part in “terrestrial-style” feeding (i.e., it bit down on its prey, rather than suction-feeding like most fish). It is the fusions in the creature’s pelvic girdle, a development that allowed it to bear more weight than its ancestors, that is most significant in tetrapod evolution. But, because it still couldn’t survive on land, the legs must have evolved for some reason other than walking.

Known Ichthyostega fossils range from 367-362.5 Mya. It appears to have had a more fish-like skull than its contemporary Acanthostega but likely relied more on its lungs for breathing than its gills. Its more robust girdle, more supportive ribs, and stronger vertebrae would seem to make it more suited for living on land than Acanthostega. It had seven digits on each hind limb (unknown for forelimbs) and fin rays on its tail. While the adult Ichthyostega was able to pull itself onto land and possibly sun itself for awhile, its hind limbs were apparently too weak to allow for much terrestrial locomotion. It had to return to the water to cool down, feed, and reproduce. [Note: Recent studies published in 2009 on the pattern of muscle attachment processes on limb bones suggest Ichthyostega was earlier in the evolutionary chain than Acanthostega. Yet, the latter was entirely aquatic….]

Other candidates for earliest tetrapod are the Elginerpeton and Obruchevichthys, both of which are known by only a few fossil bones (just a jawbone for the latter) dating to around 375 Mya.

That’s where things stood for many years….

Continued at Those Dang Tetrapod Tracks (Part 2).

Do you remember the introduction to the world of “Ida” this past May? The media frenzy and hype around this small, 47 million-year-old primate went far beyond the usual, even when it comes to the intriguing & controversial issue of human evolution. In addition to publication of the scientific paper itself in PLoS ONE, within a week’s time there was the public “unveiling” at the American Museum of Natural History by NYC Mayor Bloomberg, coverage by People magazine, debut of an official Ida web-site, a book (The Link: Uncovering Our Earliest Ancestor), and a TV special narrated by Sir David Attenborough.

Here’s what the fuss was about….

Many primatologists & paleoanthropologists postulate that, roughly 50-60 million years ago, the original Primate gave “birth” to two Suborders — the Haplorhini (or, “Dry Noses”) and the Strepsirrhini (or, “Wet Noses”). (There’s at least one alternative “school” that breaks them out a little differently, but we’ll stick with this one, for now.) The haplorhines, in turn, gave rise to various branches that produced tarsiers, monkeys, apes, hominids, and eventually modern Man. Meanwhile, the strepsirrhines didn’t change much, leading to today’s lemurs, lorises, & bush babies.

What got people excited was the fact that, while she does have many lemur-like qualities, Ida does not have certain other physical characteristics that would normally place her among the strepsirrhines. Therefore, they concluded, she must be a primitive haplorhine — perhaps even the first! This would make her an evolutionary ancestor to humans, or, at least, a distant cousin.

So, the 95%-complete skeleton became the latest find touted to be a “missing link” in the evolutionary development of Man and “clear proof of Darwin’s theory of evolution”. As Attenborough put it, Ida is “a link between the apes, monkeys and us with the rest of the mammals and ultimately the whole animal kingdom. I think Darwin would have been thrilled.” Lead researcher Jorn Hurum (who named the fossil after his daughter) said, “This fossil rewrites our understanding of the evolution of primates.” At least one paper even called her the “eighth wonder of the world.”


Ida fossil & her x-ray

But, other experts were either surprisingly mum or expressed concern over the hyperbole. Professor John Fleagle of Stony Brook University in New York challenged the claims, cautioning that the paper’s full significance would not be known until other scientists had a chance to properly examine the findings and engage in extensive debate.

The August 2009 issue of the august (pun intended) journal Science featured a review of the Ida book called “Much Hype and Many Errors” by paleoanthropologist Richard F. Kay of Duke University. Noting the sensationalistic writing style in several chapters, Kay laments, “Ida is a reprise of the seamier side of paleontology: the exploitation, with the complicity of major museums, of our natural heritage for profit.” He goes on to point out many inaccuracies and misleading statements, hinting the many errors were largely the result of a fame-seeking carelessness in rushing the book to press in time for the media blitz. Kay then questions the paleontological classification of the fossil by Philip Gingerich, University of Michigan – Ann Arbor, concluding “So [Ida] cannot be both an adapoid and a monkey ancestor. [. . .] So, Link‘s premise, that Ida is our ancestor, is fallacious. Ida is a lemur.”

And so the controversy went….

Now, it seems, the final nail may have been put in the coffin of the Ida-as-missing-link theory. Stony Brook University’s Erik Seiffert recently led a team of primatologists and paleoanthropologists that analyzed and compared 360 specific anatomical features of 117 primate species (living & extinct), including another recent (though less complete) specimen, Afradapis longicristatus, from Egypt. The goal of the study was to construct a decisive “family tree” for the Primate Order. Actually, most of the work was done a few months ago, but they put off publication until they had a chance to examine & compare Ida, too. The just-out issue of the journal Nature contains their findings.

As reported by the Associated Press, “The scientists who formally announced the finding said they weren’t claiming [Ida] was a direct ancestor of monkeys, apes and humans. But they did argue that it belongs in the same major evolutionary grouping, and that it showed what an actual ancestor of that era might have looked like. The new analysis says [Ida] does not belong in the same primate category as monkeys, apes and humans. Instead, the analysis concluded, it falls into the other major grouping, which includes lemurs.” (Sound familiar?)

More precisely, while all agree that Ida represents a new genus & species of early primate (Darwinius massillae), it now seems that she does indeed belong in the haplorhine lineage, though as part of a small side-branch that quickly went extinct. Of course, this means she cannot be a “missing link” to humans. Another very important point is that those characteristics Ida shares with the higher primates must have appeared separately from the anthropoids. This is yet another case of “convergent evolution” — something that appears to have happened quite a bit over the eons but is not supposed to be possible within the neo-Darwinian paradigm. How embarrassing!

Anthropology professor Eric Sargis of Yale is “not surprised” at the results of the Seiffert team’s analysis. The University of Toronto’s paleoanthropologist David Begun says this “confirms what most scientists think.” For his part, Hurum seems unfazed, saying “we are happy to start the scientific discussion” about where Ida & her kin fit in the primate “family tree”.

Why do I bring up all of this controversy over Ida, anyway? Well, it is NOT to cast aspersions upon scientists in general or evolutionists, or to accuse anyone of duplicity. But there are a few lessons to be had from all this, reminders of things we should always keep in mind.

1) Scientists are human, too. (Heh!) OK, what I mean is that, despite the objective nature of the scientific process, the people doing the science still have biases to deal with. It may be a particular philosophy of and approach to science; or, favoring a particular (personal?) model/theory; or, pressure to publish a paper or make sufficient progress to keep their grant; etc. These can lead to hopeful, yet hasty, conclusions, however rational they may seem at the time.

2) A certain amount of “talking up” one’s research is to be expected and is generally accepted within the scientific community. In order to justify funding, obtain desirable positions and other opportunities, etc., one must show progress, publish papers, and get people excited about what they are doing. Sometimes doing this amongst fellow researchers and academics is sufficient; but, sometimes they need a little boost, so they take it to the public via the media. (NASA and others in space exploration do this all the time.) Unfortunately, this can be taken too far, as with the case of Ida. Which leads to our third lesson….

3) As exemplified by Professors Fleagle and Kay’s comments above, the flap within the scientific community centers on two separate-but-related things. First, the lack of detailed documentation to support the initial researchers’ interpretation of the data presented in the paper, book, and documentary. As pointed out by freelance science writer & blogger Brian Switek: “Even though the authors of the paper constructed a very simple cladogram, they did not undertake a full, rigorous cladistic analysis to support their claims. I am baffled as to how they could stress the significance of this fossil without undertaking the requisite research to support their hypothesis.” From what I have read, Hurum’s supporting analysis/documentation is still a work in progress.

The other half of this is, of course, the necessity for peer review. It isn’t perfect, but the peer review process allows for more objective analysis by other scientists. Most welcome this (at least to some degree), because such critiques lead to identification of weaknesses in their theories and interpretations, which then shows them what areas to concentrate on in order to strengthen those theories. Everyone — scientists, media, politicians, and the rest of us — must recognize the need for rigorous research standards, healthy exchange of ideas, and vigorous debate.

(Ironically, though the media blitz was not nearly as great, Charles Darwin bypassed peer review when he published his book On the Origin of Species.)

The second issue is that of the questionable motives — ostensibly, profit and fame — that drove Hurum and others to sensationalize the find and push it out to the general public so quickly. As Kay said, this belies the “seamier side” of science. Some may think such a “purist” opinion is too stiff or behind-the-times or just unrealistic. Regardless, the concern that money & headlines can compromise the accuracy and objectivity of scientific research is a valid one.

4) The whole business of properly classifying the fossil record, especially hominids and humans, is all very complicated and depends on relatively few bones and a lot of inference & conjecture, with no definitive links. (I’ve probably rankled quite a few with that statement, but I’ll address the broader issue of “missing links” some other time.) Indeed, there are many different positions within the scientific community on the details of how the various species are connected. Those who try to figure it out solely through morphology (i.e., the shapes of anatomical structures), such as Seiffert’s team, often conclude one set of connections, while those who look at the genetics come up with a different “family tree”. Remember this the next time you hear or read somebody speaking as if everything is very well-known and fits together all neat & tidy.

Here’s the bottom line….

Ida is indeed a fantastic find — an amazingly preserved, near-complete skeleton of a new genus & species — that is proving to be very interesting. Her existence may even throw (yet another) monkey wrench into the reigning understanding of primate evolution. But, it is becoming clearer & clearer that she is not the much-trumpeted “missing link” that Hurum et al. would like us to think she is. At least, that’s my take on it.

I conclude with a quote from David Tyler of the Access Research Network ( “The best outcome of all this is for scientists to demonstrate more humility in handling data. So many seem to grasp at some data and brandish ‘evidence’ as though it provides a definitive answer to controversy. But this is not how data should be handled in research. Data needs to be interpreted and history shows that there is always more than one way of interpreting it. If we can all adopt a ‘multiple working hypotheses’ approach when using the scientific method, it will be progress indeed.”