New fossil proves plants are younger than previously thought


That’s not a headline you’re likely to see again. Hopefully it made you think something along the lines of “how does that work, exactly?” Because it doesn’t. If your estimate of the age of a taxon is based on its oldest known fossil, finding a newer fossil isn’t likely to change that estimate. If it’s an extinct group, a newer fossil might show that it stuck around later than you thought, but not that it originated later. Paleontologists recognize that fossil-based estimates of ages are almost always underestimates, since the fossil record is spotty (and generally spottier the further back you go).

I’m hardly the first person to point this out. For example Marshall (1990) says

…the fossil record is incomplete and first occurrences only yield upper bounds on divergence times, which may significantly underestimate true divergence times,

(BTW, paleontologists use “upper” to mean younger, since younger rock strata are generally above older strata.) Donoghue and Benton (2007) say

Palaeontological data can only provide reliable minimum constraints on lineage splitting events,

and Ho (2007) says

…fossils can only provide minimum age estimates for divergence events, as the appearance of a fossil necessarily postdates the origin of the clade of which it is a member.

So as paleontologists find more and more fossils, we should expect that estimates of the ages of most taxa will get older. It’s really the only direction they can go. Nevertheless, intelligent design proponents always seem to think that newly discovered fossils supporting older origins are evidence against evolution. For example, whale fossils pose “a challenge to evolution that won’t go away,” a fossil crustacean “turns the thumbscrews on Darwin,” and 3.7 billion-year-old microbial mats “pose an evolutionary dilemma.” Denyse O’Leary provides a long list of fossils that show that “evolution does not happen.”

Figure 5 from Bengtson et al. 2017. Rafatazmia chitrakootensis n. gen., n. sp., SRXTM renderings. (A–L) Holotype, NRM X4258. (A) Surface rendering. (B) Volume rendering with rhomboidal disks coloured for visibility. (C) Virtual slice. (D) Surface. (E) Volume. (F–L) Transverse slices (positions indicated in B). (M–O) NRM X5620, surface, volume, slice. (P–R) NRM X5574, surface, volume, slice. Scale bars 50 μm.

Figure 5 from Bengtson et al. 2017. Rafatazmia chitrakootensis n. gen., n. sp., a 1.6 billion-year-old fossil tentatively interpreted as a red alga. Scale bars 50 μm.

Most recently, David Klinghoffer thinks that 1.6 billion-year-old fossils interpreted as red algae are bad news for evolution (“Darwinism,” as he prefers to call it):

A new fossil discovery from India has pushed the history of plant-like life back 400 million years. Good news for Darwinian theory? Not at all.

His logic is familiar:

…getting from no plant-like life to red algae now evidently has 400 million fewer years to work with…Such abrupt transitions, punctuating long stasis with sudden bursts of creativity, are not what Darwin expected. However, they are what intelligent design expects, based on our own experience of how ordinary human creativity works.

So just how abrupt is Klinghoffer’s “abrupt transition”? He doesn’t specify, but a reasonable interpretation is the transition from unicellular eukaryotes to multicellular red algae (surely he’s not saying there wasn’t time for multicellularity to evolve between the origin of life and 1.6 billion years ago). The age of the first eukaryote is a subject of intense debate, with estimates ranging from less than 1 billion years to around 2.8 billion years. The earliest of those dates are impossible if the 1.2 billion-year-old fossil Bangiomorpha is really a red alga, which nearly all authors agree it is.

A recent analysis by Laura Wegener Parfrey and colleagues estimated the age of the last common ancestor of eukaryotes between 1.679 and 1.866 billion years. Let’s say, for the sake of argument, that the true age of the eukaryotes is the most recent of those estimates. That leaves ~79 million years. To put things in perspective, that’s longer than the time between the Cretaceous-Tertiary boundary (i.e. the extinction of the dinosaurs) and today. In that time, rodents diversified from one or a few species into ~1500, and bats into ~1200. Primates evolved from one or a few species into everything from mouse lemurs to mountain gorillas.

But maybe evolving multicellularity is much harder than evolving a 3000-fold range of body sizes, diverse ecologies, and complex social systems. There’s pretty good evidence that that’s not true, though. Microbial evolution experiments have shown that simple multicellularity can evolve in a matter of a few months in yeastChlorella, and ChlamydomonasLet’s put that in perspective: the time between Parfrey et al.’s youngest estimate and the new red algal fossils is eight orders of magnitude longer than those experiments.

But maybe even Parfrey et al.‘s youngest estimate is too old; maybe the origin of eukaryotes was 1.65 billion years ago, or 1.61, or 1.601, or the Tuesday before the Rafatazmia fossils were laid down. Klinghoffer’s argument comes down to two numbers: (A) the number of years between the origin of eukaryotes the origin of multicellular red algae, and (B) the number of years required for a eukaryote to evolve multicellularity. He is effectively claiming that B > A, but he doesn’t know the value of either B or A. So in the end, this is just another argument from personal incredulity: I can’t believe that this process (requiring an unknown amount of time) could possibly happen in the time available (which is also unknown); therefore, it couldn’t have happened.

 

Stable links:

Bengtson S, Sallstedt T, Belivanova V, Whitehouse M (2017) Three-dimensional preservation of cellular and subcellular structures suggests 1.6 billion-year-old crown-group red algae. PLOS Biology, 15, e2000735.

Boraas ME, Seale DB, Boxhorn JE (1998) Phagotrophy by a flagellate selects for colonial prey: a possible origin of multicellularity. Evolutionary Ecology, 12, 153–164.

Butterfield NJ (2000) Bangiomorpha pubescens n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes. Paleobiology, 26, 386–404.

Eme L, Sharpe SC, Brown MW, Roger AJ (2014) On the age of eukaryotes: evaluating evidence from fossils and molecular clocks. Cold Spring Harbor Perspectives in Biology, 6, a016139.

Hedges SB, Kumar S (2004) Precision of molecular time estimates. Trends in Genetics, 20, 242–247.

Ho SYM (2007) Calibrating molecular estimates of substitution rates and divergence times in birds. Journal of Avian Biology, 38, 409–414.

Kaźmierczak J, Kremer B, Altermann W, Franchi I (2016) Tubular microfossils from ~2.8 to 2.7 Ga-old lacustrine deposits of South Africa: A sign for early origin of eukaryotes? Precambrian Research, 286, 180–194.

Marshall CR (1990) The fossil record and estimating divergence times between lineages: maximum divergence times and the importance of reliable phylogenies. Journal of Molecular Evolution, 30, 400–408.

Parfrey LW, Lahr DJG, Knoll AH, Katz LA (2011) Estimating the timing of early eukaryotic diversification with multigene molecular clocks. Proceedings of the National Academy of Sciences of the United States of America, 108, 13624–13629.

Comments

  1. polishsalami says

    This shows how difficult it is for the human mind to grasp the immense time scales involved in our planet’s history. (It’s even worse for the anti-science crowd.)

    For instance, when I read of some event’s age being altered from 185 mya to 190 mya, my first instinct is to think this is just a minor tweak; but five million years is long period of time. Our brains weren’t designed to comprehend these things.

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