Don’t be fooled by brown algae. Kelps and their relatives could easily be mistaken for plants, with their stemlike stipes, leaflike blades, and rootlike holdfasts. Nothing could be further from the truth. You are more closely related to a shiitake than a kelp is to a kale.
Brown algae are heterokonts, so-called because they have two different (hetero) flagella in the swimming stage of their life cycle. They are about as far distant from plants as it’s possible to be as a eukaryote. The similarities between kelps and plants are due to convergent evolution rather than to common descent.
Even more surprisingly, the brown algae are much younger than plants. A new preprint by Naomi Phillips and colleagues tries to pin down just how much younger. As a preprint, this has not been through peer review, so we should treat the results as tentative.
Phillips and colleagues sequenced the mitochondrial and chloroplast genomes of a filamentous relative of the brown algae, Schizocladia ischiensis, and used the resulting data in molecular clock analyses. I’ll focus here on the chloroplast data, since the mitochondrial data give similar estimates of divergence times.
The divergence we’re interested in is marked ‘G’ in the figure above: that’s the divergence between Schizocladia and the brown algae (Phaeophyceae). The point estimate is 252.4 million years ago, but with a rather large confidence interval (the blue bar): 154.3-406.8 million years ago. The corresponding estimate from the mitochondrial analysis is 221 million years, with a confidence interval spanning 165.6-290.4 million years, pretty similar as these things go. Since this node represents the divergence between brown algae and their closest relative, it serves as the oldest bound for the age of the brown algae (brown algae could not have existed before their common ancestor with Schizocladia). The corresponding lower bound (i.e. the youngest possibility) is represented by node H, the oldest divergence within the brown algae: 116 million years (confidence interval of 66.9-208.5) in the chloroplast analysis, 136 million years (106.1-155.8) in the mitochondrial analysis.
These numbers may change by the time the manuscript makes it through peer review: it’s a complex analysis with over a dozen fossil calibrations (for which the authors should be commended, but if any of them are misplaced, the analyses will have to be redone). Nevertheless, let’s look at what these results would mean if they hold up.
Two previous estimates of the divergence between Schizocladia and the brown algae are available on timetree.org: 196 million years (confidence interval 268–131) from Brown & Sorhannus 2010, and 258 million years (228–277) from Kawai et al. 2015 (note that Kawai is also a coauthor of the new preprint).
So we have a reasonably good correspondence among the different studies: point estimates from 196-258 million years, with confidence intervals spanning 131-407 million years. It may not be very precise, but studies of this sort rarely are, and I would rather the authors acknowledge the wide range of estimates than succumb to what Graur & Martin call “the illusion of precision”. What is interesting is that none of these estimates, which, remember, represent the oldest possible origin of the brown algae, overlap with the estimated age of one putative brown algal fossil, Miaohephyton bifurcatum (550-600 million years; Xiao et al. 1998). That’s true even if we include the oldest end of the oldest confidence interval, 406.8 million years.
Both things can’t be true: either the Miaohephyton fossils are misidentified or the molecular clock estimates, all of them, are off by at least twofold. Bear in mind that while the divergence between Schizocladia and the brown algae represents the oldest possible origin of the brown algae, a fossil, by definition, represents the youngest (a fossil can never be older than the group it represents). It would be way too far outside of my expertise to try to evaluate the evidence that Miaohephyton was a brown alga. I welcome any paleontologists who run across this to weigh in in the comments. I’ll be interested to see how this shakes out.
Brown, J. W., and U. Sorhannus. 2010. A molecular genetic timescale for the diversification of autotrophic stramenopiles (Ochrophyta): substantive underestimation of putative fossil ages. PLoS ONE 5: e12759. doi: 10.1371/journal.pone.0012759
Graur, D., and W. Martin. 2004. Reading the entrails of chickens: molecular timescales of evolution and the illusion of precision. Trends in Genetics 20:80–86. doi: 10.1016/j.tig.2003.12.003
Kawai, H., T. Hanyuda, S. G. A. Draisma, R. T. Wilce, and R. A. Andersen. 2015. Molecular phylogeny of two unusual brown algae, Phaeostrophion irregulare and Platysiphon glacialis, proposal of the Stschapoviales ord. nov. and Platysiphonaceae fam. nov., and a re-examination of divergence times for brown algal orders. Journal of Phycology 51:918–928. doi: 10.1111/jpy.12332
Phillips, N., E. L. Braun, J. Boore, B. Cheda, M. P. Salomon, H. Kawai, and T. Yamagishi. 2020. Schizocladia ischiensis organellar genomes: estimating the origin of multicellularity in heterokonts and the emergence of shallow ocean ecosystems. doi: 10.21203/rs.3.rs-20417/v1