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Billion-year-old multicellular green alga recently found

lpetrich

lpetrich

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It's hard for me to find a nontechnical article on this discovery that does not contain gross mistakes. So I'll try to give a good nontechnical summary.

The journal paper -- be prepared for a lot of professional jargon
A one-billion-year-old multicellular chlorophyte | Nature Ecology & Evolution
A reprint of that paper

Fossils of a multicellular green alga were recently discovered in Liaoning Province in northern China. The individual algae are not very big: thin branching strands about 1 - 2 millimeters long. They are inferred to be "siphonocladous", an odd sort of structure that some present-day green algae have, a structure where the strands do not have cell walls, making them long "super cells" with multiple nuclei. In general, this kind of structure is called "coenocytic", and it evolved independently several times. The paper has a nice artist's conception of these algae growing on a seafloor.

This alga has been named Proterocladus antiquus, using the genus name of some similar fossil algae found in Svalbard and a new species name. Those algae were found in rocks some 700 - 750 million years old: Paleobiology of the Neoproterozoic Svanbergfjellet Formation, Spitsbergen - BUTTERFIELD - 1994 - Lethaia - Wiley Online Library While the Svalbard specimens are fragmentary, the new one's paper lists 1028 specimens of this new one, and shows pictures of several complete ones.

The fossils' containing rocks are dated at around 1 billion years. This is comparable to the age of the oldest known multicellular eukaryote, the red alga Bangiomorpha pubescens, from 1.047 +0.013 -0.017 billion years ago (Precise age of Bangiomorpha pubescens dates the origin of eukaryotic photosynthesis | Geology | GeoScienceWorld).

So we have a billion-year-old multicellular red alga and a billion-year-old multicellular green alga.
 
The paper's authors interpret this alga as having features much like some present-day filamentous green algae:
To the best of our knowledge, modern siphonocladalean chlorophytes provide the most appropriate morphological analogue of Proterocladus. Among uniseriate filamentous chlorophytes, siphonocladous construction is most common in the class Ulvophyceae, particularly in the order Siphonocladales (=Cladophorales, and is the preferred name for this group of chlorophytes according to ref. 50). Importantly, the initiation of lateral branches as outpocketing structures always subjacent to a septum is a key feature among extant siphonocladaleans such as Cladophora and Rhizoclonium (Fig. 3d). Indeed, in addition to siphonocladous construction and the unique branching pattern, Proterocladus also shares with Cladophora and Rhizoclonium a number of other morphological features, including a holdfast and an epibenthic habit, intercalary cell division with centripetal invagination (as indicated by constriction septa), as well as differentiated cells with lateral pores probably representing sexually reproductive cells (Supplementary Table 1).Thus, among all morphological analogues discussed above Proterocladus compares best to siphonocladaleans and their morphological similarities are suggestive of a phylogenetic relationship Of course, we cannot rule out the possibility that Proterocladus may represent an extinct group of siphonocladous eukaryotes that independently evolved a siphonocladalean-style branching pattern, but Occam's razor leads us to hypothesize that Proterocladus is a possible siphonocladalean chlorophyte.
Click to expand...
In simpler language, this alga has several structural features much like what some present-day algae have, though the authors do not feel confident enough to rule out convergent evolution.
 
Implications for the evolution of algae are rather obvious. Bangiomorpha and Proterocladus are identified as a red alga and a green alga, each one with separately-evolved multicellularity. Proterocladus, though a green alga, is only distantly related to land plants.

Eukarya is currently divided into several superkingdoms, with one of them, Archaeplastida, being descended from the organism that had the first chloroplast, a cyanobacterium that took up residence inside of it.

Archaeplastida:
  • Viridiplantae / Chloroplastida - green algae
    • Chlorophyta - some green algae, Proterocladus
    • Streptophyta - some green algae, land plants
  • Rhodophyta - red algae, Bangiomorpha
  • Glaucophyta - some obscure one-celled algae with its chloroplasts having a cyanobacterium-like cell wall.


The highest-level relationships of eukaryotes are determined with molecular techniques, and they often have little or no phenotypic support. The New Tree of Eukaryotes - ScienceDirect discusses where we are now. Several large groups and lots of "orphan" organisms, organisms inferred to have branched off when the large groups branched off from each other.

That's also the situation with prokaryotes - several groups mainly defined with molecular criteria, and with very limited phenotypic support.
 
Here are the well-supported groups:

  • TSAR - telonemids, SAR: ((stramenopiles, alveolates), rhizaria)
  • Haptista - haptophytes, centrohelids
  • Cryptista - cryptomonads, katablepharids, Palpitomonas
  • (Chromalveolata is broken up)
  • Archaeplastida - Chloroplastida, Rhodophyta, Glaucophyta - Its members have primary plastids (direct from some cyanobacterium). But it emerges very weakly, and Cryptista branches very close -- or inside of it.
  • Amorphea - Amoebozoa, Obazoa: (breviates, apusomonads, Opisthokonta: (animals, chanoflagellates, fungi))
  • CRuMs - collodictyonids (= diphylleids), Rigifilida, Mantamonas
  • Discoba - Euglenozoa, Heterolobosea, Jakobida, Tsukubamonas
  • Metamonada - has organisms like Giardia and Trichomonas
  • (Excavata is broken up)
  • Hemimastigophora
  • Orphan Taxa - has organisms with uncertain placement like Ancoracysta, Picozoa, malawimonads, and ancyromonads, all free-living protozoa. Ancoracysta may be close to Haptista and malawimonads to Metamonada, though they may also be early divergers like the Hemimastigophora
Some possible larger relationships:
  • Haptista, TSAR
  • Cryptista, Archaeplastida
  • Diaphoretickes: TSAR, Haptista, Cryptista, Archaeplastida
  • Amorphea, CRuMs
The location of the root of the eukaryote tree has been rather difficult to pin down.
 
As to what the Earth looked like back then, the continents were forming the supercontinent Rodinia, after originating as fragments of an earlier supercontinent, Columbia (Nuna).

The atmosphere has much less molecular oxygen than today, something like 10%. It would not have been safe for us to breathe.

There were no land plants, though there were likely plenty of soil microbes like cyanobacteria. So the land would look very barren, even in very wet areas.

 Terrabacteria is a clade of prokaryotes that includes Gram-positive ones: (Actinobacteria, Firmicutes), Cyanobacteria, Chloroflexi, and Deinococcus-Thermus.

Gram-positivity is testing positive for a certain stain that reacts with some thick cell walls. That is likely an adaptation for dryness. Another adaptation is evident in Deinococcus radiodurans, an organism that can survive huge doses of ionizing radiation. It does so with hyperactive gene repair, something that helps it survive dryness.
 
 Acritarch - hollow organic microfossils, often with spines or hairs on them.

Palaeos Proterozoic: The Proterozoic Era has an article on acritarchs, with a picture that shows their increasing diversity over time. Not much variety when we first see them, some 1700 Mya (million years ago), but a lot of variety after 900 Mya.

Acritarchs are usually concluded to be single eukaryote cells, including one-celled organisms. But it is much more difficult to identify their affinities than for organisms like Bangiomorpha and Proterocladus.


I looked for some good introduction to fossil biomarkers, but I couldn't find any. Stuff like hopanes and steranes and the like.
 
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