On the Origin of Animals

ResearchBlogging.orgThink of an animal – any animal you’d like. Unless you’re a big fan of sponges or jelleyfish, or a protistologist perhaps, you likely thought of an animal that belongs to the Bilateria. This group includes all animals that show bilateral symmetry, so every single insect, vertebrate and mollusk belongs to this group. Like in any large family, some members don’t seem to fit in, like the radial starfish and sea urchins. But on closer inspections, these species are bilaterally symmetrical in their larval stages. The most recent common ancestor of all Bilateria is expected to have lived somewhere between 600 and 550 million years ago. This creature probably had many of the complex features seen in bilaterals today, like a centralized nervous system, internal organs and a blood circulation system. But where do we have to look if want to reconstruct the traits of this ancient ancestor? As this ‘Urbilaterian’ probably didn’t leave any fossils behind, scientists have to look in other places to find their evidence. In a Nature paper published last month, a team of researchers used the conserved expression of mircoRNAs to piece together some information about this great-great grandmother of animals.

MicroRNAs are short strands of RNA that have the ability to silence genes, by binding to the mRNA molecules in a complementary fashion. Think of a microRNA in this way: some genes that play a central role in neuronal cells, are useless for cells that make up the gut. To preven the neuronal genes from being translated in the gut, the cells silence them by expresssing a single set of microRNAs that bind the neuronal gene transcripts. Since this happens in all tissues, it’s safe to say that microRNAs play a central role in the establishment and maintenance of diverse tissue types. The evolution of Bilateria coincides with the evolution of many complex tissue types in this group of animals, so microRNAs have the potential to be a great source of evidence for the evolution of this large group of animals.

Foteini Christodoulou and colleagues analyzed the localization of a whole bunch of microRNAs in a wide variety of species, and infer some characteristics of the ‘Urbilaterian’. The team performed these analyses in the all star team of animals above. The only species that does not belong to the Bilateria is the sea anemone Nematostella you see in the bottom left. The top two creatures are marine worms (annelids, to be specific), which are suspected to have retained many ancestral bilaterian features. In the bottom right we find the sea urchin, Strongylocentrotus, which hides its bilaterism very well, as described in the introduction. You can see how these organisms are evolutionary related in the phylogenetic tree below.

Phylogenetic tree of a sea anemone, sea urchin and two annelid worms. Tree generated using itol.

The paper contains several interesting experiments and conclusions, based on localized microRNA expression. Some microRNAs appear to have evolved early on in bilaterian evolution. In the larval forms of both worms and the sea urchin, miR-29, miR-34 and miR-92 were only expressed in cilia cells that are used for locomotion. This set of microRNAs is really old, as the most recent common ancestor of sea urchins and annelids lived just after the split between bilaterals and other animals (in fact, sea urchins and annelids span the protostome / deutorostome divide). In another example, the team analyzed the localization of miR-9 and miR-9*. In mice, both microRNAs are found in the olfactory brain centres that are responsible for processing smells. In one of the worms, miR-9 and miR-9* localized to the base of of a pair of antenna. One of the functions of these antenna is to sense chemicals in the environment of the worm. So in species as distantly related as mice and annelids, this microRNA pair localizes to specialized cells involved in the processing of chemical information! This feature has been retained over the past 500 million years.. pretty cool stuff!

Localization of three different microRNAs in Platynereis (annelid worm). Picture taken from reference.

The team found microRNAs that were more or less specific for almost any tissue: gut, muscles, central nervous system, sensory organs, you name it! Aside from generating some beautiful images (see above), this also means that microRNAs played a very important role in the establishment of a wide variety tissues. The fact that some of these localizations have been conserved over hundreds of millions of years only highlights in their importance. In that sense, microRNAs seem to deserve a place alongside HOX genes as robust and conserved signals of animal evolution. Identifying more microRNAs, their localization and their gains and losses in evolution will allow researchers to gain deeper insight into why animals are the animals that they are today.

After all – I’m still an animal.

Christodoulou, F., Raible, F., Tomer, R., Simakov, O., Trachana, K., Klaus, S., Snyman, H., Hannon, G., Bork, P., & Arendt, D. (2010). Ancient animal microRNAs and the evolution of tissue identity Nature, 463 (7284), 1084-1088 DOI: 10.1038/nature08744

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9 comments to On the Origin of Animals

  • I’ve gotten some flack for this, but I think spherical geometry is useful in determining the deep ancestral life forms, such as the icosahedral viruses and primitive pentadactyle/pentameric traits common to so many taxa. Rather than seeing simple bilaterals and radial segmented worms as very early, I view them as very derived, just as “simple” snakes are derived from lizard-like ancestors, not vice versa. AFAIK all life goes through a reproductive spherical stage, egg/spore/seed, no matter what external form the egg case, larvae & adults eventually become. A sphere must have certain chemical/physical properties, whether at the nano-scale micro-scale or macro scale, otherwise it dissipates/breaks at the wrong time, and this preceded more advanced development of polarization, protein & lipid layers, later mineral shells, locomotion, complex feeding etc.

    “some genes that play a central role in neuronal cells, are useless for cells that make up the gut.”

    If you google ‘serotonin’, you’ll find that while it is a brain precursor to melatonin (sleep), most is produced in the gut, where it is sent via platelets to transform bone, which is just a mineral reservoir. Animals are complicated, tending to re-use systems in new ways when advantageous.

    • Thanks for your comment!
      Considering spherical organisms more ancestral is a view I didn’t come across before. It’s certainly true that all bilaterals go through a spherical blastula stage. But as soon as the mouth and anus develop, directionality is introduced in the embryo, in the form of the anteroposterior axis. Being able to discriminate up from down is also a major advantage, perhaps explaining the origin of the dorsoventral axis. The fact that all radial organisms go through a bilateral larval stage also seems to suggest that the bilaterian ancestor was a bilateral as well.
      Many of the earliest metazoan fossils from the Ediacaran biota also show segmented body plans, instead of radial ones. Maybe if we go further back, to the origins of multicellularity, that we will find our radial ancestor?

      And funny that you should mention serotonin and melatonin, I recently came across the example you write while working with the kegg pathway database! And you’re of course right in stating that parts (biochemical or genetical) get re-used, mixed and matched all the time. My short introduction to microRNAs is wholly inadequate of course, as it is a subject that deserves an entire blog itself! (after writing this down, I looked it up whether one existed.. and there we go! Seems more focussed on diseases though..)

  • Dunbar

    To be a pedant, technically not all blastulae are perfect spheres. I believe, for instance, chicken blastulae are discs (or flattened spheres).

    I’m a bit confused about DDeden’s comment– does he/she mean to say that an apolar nature precedes, historically and ontogenetically, a polar nature to animal life? If so, I find that odd because as all cell biologists know, asymmetrical cell division and therefore polarisation is immensely important to all cellular life. An egg, for instance, is instantly polarised upon fertilisation. Yeasts have been shown to differentially pass on proteins to daughter cells– bacteria I think do something similar. Heck, tons of cells (and viruses) come in all sorts of weird shapes. There’s no validity in saying that spherical geometry was important for ancestral forms without more convincing arguments, especially when life seems to like polarity.

  • I don’t know of any of these which do not go through a spherical stage sometime during the reproductive cycle:

    Like a pin pricking a bubble, conception induces polarization (asymmetry), followed by subdivision into blastula & growth into ‘all sorts of weird shapes’.

    Before that, the pre-fertile egg cell external membrane has localized spherical symmetry (simplest, max. volume with min. surface area, max. packing density) where no axis is predominant; the only membrane “polarities” are inside/outside (osmotic) and structural (geodesic) which are in isotopic equilibrium until conception induces nonspherical (complex) symmetry.

  • [...] at Thoughtomics admits he is an animal.  In his post, On the Origin of Animals, he discusses a Nature paper published last month by a team of researchers that used the conserved [...]

  • [...] of tissue identity and offer some clues as to what the Urbilaterian was like. Lucas bloged about this a while ago in [...]

  • DDeden

    6 Elements (CHONSP) synergise to form the biological core of life, 6 elements (ROYGBIV) prismatically synergise to form the electromagnetic core of light, 6 hoop elements synergise triaxially to weave a paper (truncated icosa) sphere.

    I was just reading about some flat square archaebacteria from Sinai
    living in hypersaline conditions which have peripheral vacuoles/
    bubbles/vesicles, thought either for buoyancy (see air sacs) or
    osmosis/sunlight/respiration. There Magnesium is unusually abundant. Also noted triangle flat ones (japonica) which become spheres when salt is reduced.



    Human stem cells become either fat cells or bone cells depending solely on their exterior environment (epigenetics)

    In the April issue of Developmental Cell, the Hopkins researchers
    report that mesenchymal (pronounced mez-EHN-kih-mal) stem cells forced
    to be spherical efficiently transform into precursors to fat cells,
    while those allowed to stretch and flatten move closer to becoming
    bone cells.

  • [...] The marine ragworm, or Platynereis dumerilii, has recently come in vogue for studying how eyes and organs evolved in animals. Many of its characteristics, like its large antennae and sensory palps, seem to [...]

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