The return of the giant lichens?

Gosh, can someone tell me if this is bullshit or if he has a point? O.o

It’s rather annoying when a paper comes out that basically threatens to turn what you think you know on its head, and you’re simply not equipped to evaluate its claims. This is the case with Retallack (2012). I’m fascinated by early animals, and endlessly bewildered by the strange fossils of the late Precambrian. While I’m aware that Ediacaran fossils have been interpreted as everything from microbial mats through animals to giant protists, I had the impression that the non-animal interpretations of iconic fossils like Dickinsonia, Spriggina, Parvancorina or Charniodiscus have slowly retreated to the fringe in the decades since their discovery.

And now this guy, whose name I’ve heard enough times to pay attention, gets into Nature arguing that the namesake formation of the Ediacaran period actually originated on dry land, and the iconic fossils are preserved in a manner more like plants, fungi or lichens than animals.

The paltry one semester of introductory geoscience I did years ago is nowhere near enough to comment on all the stuff he says about soils and microbial mats and preservation. I feel completely out of my depth, rocking precariously at the mercy of the waves…

Obviously, this assessment of the original Ediacara site doesn’t affect every fossil site from the period. The latest Precambrian reefs of the Nama Group remain marine reefs containing the remains of unknown animals that grew some of the first mineralised skeletons.

My big question at the moment is how Retallack would interpret the preservation of the White Sea assemblage. This contains similar kinds of fossils to the sites he’s reinterpreted as terrestrial. There’s Dickinsonia and several others like it, there’s Parvancorina, there’s Cyclomedusa*. And this is where hundreds of specimens of my Platonic love Kimberella come from, often associated with crawling and feeding traces. That guy moved around and grazed – plants and lichens seldom do such things! So was Kimberella a land animal? That would be the biggest palaeontological sensation of the decade if not the century. Or did dickinsoniids etc. occur both on land and underwater? Or did the White Sea fossils span a wide variety of environments? (I’m not sure about the distribution of the various White Sea fossils relative to each other…)

Oh my. I wonder what will come out of this. Publication in Nature makes it dead certain that any expert who’d vehemently disagree will find the article. Let’s pull out the pop corn and watch…

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*It’s slightly odd that he seemingly treats Cyclomedusa and other “medusoid” fossils as though most people considered them jellyfish. That may have been their original interpretation, but I thought it was widely discredited now.

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Reference:Retallack GJ (2012) Ediacaran life on land. Nature advance online publication available 12/12/12, doi:10.1038/nature11777

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News bites

Just quickly before I completely forget about these…

(1) Common ancestry of segmentation: back-and-forth-and-back-and-forth

Seaver EC et al.(2012) Expression of the pair-rule gene homologs runt, Pax3/7, even-skipped-1 and even-skipped-2 during larval and juvenile development of the polychaete annelid Capitella teleta does not support a role in segmentation. EvoDevo 3:8

I’ve made throwaway mentions of segmentation before. The conundrum about segmentation is whether (or rather, to what extent) it is homologous in the three “eusegmented” phyla, arthropods, annelids and chordates. It arises because all three phyla are separated from the others by many lineages that aren’t usually considered segmented – yet the three share some tantalising similarities. People have been trying to solve the question by comparing the genetic mechanisms generating the segments in each group, with mixed results. One of the papers in my previous news bite post was about the similarity of segmentation in arthropods and vertebrates. Now, here’s one for the differences between arthropods and the wormies. (You can’t say I’m not fair :-P) The genes listed in the study’s title were originally described in the fruit fly Drosophila melanogaster, one of the best studied animals in developmental biology (and, like, every other area of biology). There, they have an interesting role in that each of them helps define every other body segment. IIRC, Pax3/7 (known in flies as paired) and even-skipped are for even-numbered segments, runt is for the odd ones. Now, segmentation in Drosophila is (to put it mildly) fucking weird, but if memory serves, several of these pair-rule genes have been confirmed to play similar roles in less eccentric arthropods. Elaine Seaver and colleagues looked at their expression in their favourite worm (this guy. Seaver’s group obviously didn’t pick it for its beauty :-P), and they found that they were active in… nothing resembling a two-segment pattern. Or anything segment-related. The more genetic studies come out, the more complicated the whole segmentation issue is looking…

(2) Someone found the cause of the Cambrian explosion. (Again.)

Peters SE & Gaines RR (2012) Formation of the ‘Great Unconformity’ as a trigger for the Cambrian explosion. Nature 484:363-366

The Cambrian explosion is probably not what you think it is (no, all animal phyla didn’t just suddenly pop into existence fully formed ;)). Nevertheless, the (relatively) quick rise of animals – particularly animals with hard parts – beginning in the Early Cambrian is still odd enough to fascinate generations of palaeontologists, evolutionary biologists and geologists. The list of proposed causes is pretty long by this point (and believe me, I really really would like to go into them once… but, uh. Huge, dauntingly huge topic). Explanations range from denying the need for an explanation through pinning it on oxygen levels, ice ages, developmental genetics, predation, biomineralisation, even the evolution of eyes, and, working from memory, I probably left some more out of that list. Peters and Gaines’ preferred explanation seems to be geological and ecological: they suggest that a combination of lots of erosion/weathering on land, and a subsequent rise in sea levels, led to large new shallow seas that were chock full of dissolved minerals. New habitats to conquer + widely available minerals = an explosion of new animals with mineralised hard parts. This study is a nice two-in-one: it purports to explain not only the Cambrian explosion, but also the conspicuous gap in the geological record that separates Cambrian from Precambrian rocks in many places.

Back to the warm little pond?

Oh, look, an abiogenesis post again! This time, of pure squee.

Back in olden days, a certain Charles Darwin wrote to a dear friend:

“It is often said that all the conditions for the first production of a living organism are now present, which could ever have been present. But if (and oh! what a big if!) we could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, &c., present, that a proteine compound was chemically formed ready to undergo stillmore complex changes, at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed.”

The famous quote is obviously not a theory of abiogenesis as such; rather, it’s some (fairly obvious, in hindsight) reasoning as to why life doesn’t originate anew today. Still, the warm little pond has become something of a symbol for abiogenesis.

Ironically, it doesn’t seem to enjoy much popularity in the field itself.* The scientist favourites are more along volcanic lines – hydrothermal vents of some stripe, or cold seeps on the seafloor. The sea is a common theme with these… but volcanic environments spewing all kinds of useful chemicals are not limited to the sea. In fact, Mulkidjanian et al. (2012) argue, much more abiogenesis-friendly volcanic sites might be found on dry land.

Living cells, the reasoning goes, contain far higher concentrations of potassium, phosphate, zinc and some other ions than any ocean anywhere (and, most likely, anytime) on the planet. The most ancient proteins that are still present in living things today don’t simply tolerate these ions – they depend on them. However, the first cells would not yet have possessed the tightly insulating membranes and armada of ion-shuttling proteins modern cells use to maintain their special salt cocktail in all kinds of environments. Therefore, cells must have arisen in a salt solution similar to their interiors. (Note that this reasoning doesn’t necessarily apply to the origin of replicating molecules. We’re not talking about the very first steps towards life just yet.)

Where might one find such an environment? Not anywhere in the sea, the researchers argue. “[H]igh concentrations of transition metals, such as [zinc] and [manganese], are found only where extremely hot hydrothermal fluids leach metal ions from the crust and bring them to the surface,” they remind (pE823). Now, this also happens at deep sea vent systems – but deep sea vents don’t spill out much phosphate, so they only contain part of the “life cocktail”. Geothermal fields on land, on the other hand… Geothermal regions where the hot water boils and steams through the rock are also potassium-rich (again, unlike any place in the sea), bringing all the essential ingredients together. The steam either boils all the way to the surface and then condenses in pools, or reach the surface as a liquid and result in nice, bubbling mud pots.

These environments today are very acidic, which is not generally good for life – however, their acidity is the result of hydrogen sulphide reacting with oxygen. There was very little oxygen in the ancient atmosphere, so the first protocells forming in a primordial mud pot or hot volcanic pool wouldn’t have to contend with that little nuisance called sulphuric acid. What’s more, the silica carried by the rising steam would precipitate as silicate minerals of the exact sort that are thought to be conducive to the formation of all kinds of protocelly things including primordial lipid membranes and nucleic acids (Hanczyc et al., 2003)!

Swell! And there are other little gems scattered throughout Mulkidjanian et al. (2012). For example, some of the minerals that form in a geothermal environment without oxygen can catalyse the formation of various organic compounds, which the early protocells could eat. Inland geothermal fields also live much longer than deep sea vents, providing more time for life to get off the ground. What’s more, the rocks wetted by condensing geothermal vapours would dry out and then be re-wetted many times, which concentrates some of the organic chemicals thought to be most important for the origin of key life molecules like nucleic acids. Even better, geothermal vapours are full of borate, which can stabilise ribose – a component of RNA, thought to be the substance of the first genomes.

So, this study envisions a field of geysers, mud pots and warm pools fed by the runoff as the cradle of life. Probably not quite what Darwin had in mind, but hey, when Darwin thought up his warm little pond, people thought proteins were the substance of inheritance. I think we can give him a little slack here 😉

We’ll see what other abiogenesis researchers have to say about the idea, but for the time being, I rather like it!

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*Disclaimer: my reading on abiogenesis mostly consists of fangirling over interesting new ribozymes and cdk007’s Youtube videos. It’s not a field I would call myself well-versed in. So what “seems” to me might just be a nosy outsider’s distorted impression 😛 (Nonetheless, the paper seems to express the same impression, and the authors ain’t newbies to this field. Well, Eugene Koonin, the principal investigator, certainly isn’t.)

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References:

Darwin’s letter to Joseph Hooker is recorded in Darwin F (ed.) (1887) The life and letters of Charles Darwin, including an autobiographical chapter. London: John Murray. Volume 3.

Hanczyc MM et al. (2003) Experimental models of primitive cellular compartments: encapsulation, growth, and division. Science 302:618-622

Mulkidjanian AY et al. (2012) Origin of the first cells at terrestrial, anoxic geothermal fields. PNAS 109:E821-E830