Being multicellular does not necessarily qualify a fossilised organism as being a member of the eukaryote domain: such a classification is assured when there is strong evidence for many cells constituting a functional whole with specialised parts. That eukaryotes also have cells with nuclei and a variety of organelles is a prerequisite for living members, though such evidence is extremely rare and disputed for fossils, and the earliest convincing examples are from 1700 Ma sedimentary rocks. Using a molecular clock approach to the differences in genetic make-up between modern eukaryotes might seem one means of estimating when the last common ancestor of all of them lived, but the Catch-22 is having incontrovertible examples from the distant past as means of calibrating that approach. A fourth possible ‘fingerprint’ is the presence of biomarker chemicals in sedimentary rocks that are exclusive to living Eucarya, steranes derived from sterols being an example.
Since the 1970s the oldest candidate for eukaryote status has been a coiled form a few centimetres across made from a strap-like carbon film, known as Grypania that some regard as a primitive alga. Yet it could equally be a colonial bacterium. Grypania are know as far back as those found in the 1900 Ma ironstones of Michigan, USA. Thin black shales from a mixed marine and terrestrial sequence of 2100 Ma siltstones and sandstone in Gabon, West Africa now provide something far more spectacular (El Albani, A. and 21 others 2010. Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago. Nature, v. 466, p. 100-104). They are complex and look a little like an irregular discus 1-2 cm across. Being replaced by fine grained iron sulfide they preserve odd internal structures discernible using X-ray tomography – folds of their central node – signs of flexibility in the original material – and scalloped flanges with radial slits. To the authors this suggests coordinated growth rather than the amorphous characteristics of bacterial biofilms such as stromatolites. They are completely unlike any living colonial bacteria. Their host rocks have yielded steranes characteristic of eukaryote biochemistry, but contamination from groundwater cannot yet be ruled out.
Only the one and a half billion year younger Ediacara fauna comes close in terms of complexity of form to the Gabon fossils. Yet whether they are the earliest-known eukaryotes or bacterial colonies whose growth was coordinated between the cells of which they were composed by some unknown exchange of information cannot be pinned down. However, their age is appropriate for the rise of the oxygen-demanding Eucarya, a few hundred million years after the start of planetary oxygenation. Perhaps more important, the surprising find will give palaeobiologists the impetus and confidence that large body fossils can indeed be found in the Palaeoproterozoic Era.
See also: Donoghue, P.C.J & Antcliffe, J.B. 2010. Origins of multicellularity. Nature, v. 466, p. 41-42.
Ordovician lagerstätte in Morocco
It was during the Ordovician Period that multicelled life really took off (see The Great Ordovician Diversification in the September 2008 issue of EPN), but the fossil record seems to suggest that the wonderfully diverse soft-bodies fauna of the Cambrian, exemplified by that from the Burgess Shale, didn’t survive to take part. It turns out that this may be an artefact of imperfect preservation, for a Lower Ordovician equivalent of the Burgess Shale has been unearthed in Morocco (Van Roy, P. et al. 2010. Ordovician faunas of Burgess Shale type. Nature, v. 465, p 215-218). It is just as rich and even shows more organic detail, highlighted in reds, oranges and yellows because iron sulfide that mineralised soft parts has since been gently oxidised. A fascinating link with the Burgess Shale is that the fossil taxa from the Moroccan lagerstätte are related to those in that most famous Middle Cambrian rock unit.
On the subject of exceptionally preserved fossil material, one of the Burgess Shale oddities, new specimens of Nectocaris pteryx allow a detailed reconstruction. What a stunning beast! This 5 cm stem-group cephalopod had two tentacles, enormous stalked eyes and a funnel shaped device that may have been for squid-like jet propulsion. Reconstruction of its back-end suggests a cuttlefish-like means of propulsion by a flap of tissue around the main body, but with no sign of any stiffening ‘bone’.
Possible abiotic mechanisms for DNA splitting and cell membranes
The central feature of the DNA helix is its ability to ‘unzip’ and recombine as part of the replication that is essential for all known living things. In doing this, DNA copies itself. It is one thing to deduce this from DNA’s structure and the meiotic aspect of reproduction, but quite another to figure out how it might have arisen. An experiment that mimics conditions in porous sea-floor lava – a temperature gradient and small-scale convection inside a capillary tube – shows that this lengthways splitting does occur on the hot side of the gradient. On the cool arm of the convection the halved ribbons of DNA reassemble (Mast, C.B. & Braun, D. 2010. Thermal trap for DNA replication. Physical Review Letters, v. 104, p. 188102-188105). This is a long way from life’s origin and even that of DNA as an isolated entity without a cell, but perhaps one step towards a better understanding of both. It seems pretty certain from a range of evidence – e.g. heavy metal centred proteins and heat-shock proteins – that life sprang from physical and chemical processes around hot vents on the ocean floor. What’s next on the experimental agenda: membranes to bag-up genetic material such as DNA as a precursor to the cell? It’s been done (Budin, I. et al. 2009. Formation of protocell-like vesicles in a thermal diffusion column) using fatty acids that are relatively easy to generate abiotically. Some can transform into flexible membranes that curve in on themselves – amphiphiles – and vesicles of these formed in Budin et al.’s capillary tubes.
See also: McAlpine, K. 2010. Life cooked up in undersea cauldrons. New Scientist, v. 206 (29 May 2010 issue), p. 14.