On Tuesday, June 1, 2021 at 8:12:43 AM UTC-4, John Harshman wrote:
> On 5/31/21 3:15 PM, Peter Nyikos wrote:
> > On Saturday, May 29, 2021 at 9:16:28 AM UTC-4, John Harshman wrote:
> >> On 5/28/21 7:07 PM, Peter Nyikos wrote:
> >>> Over in talk.origins, there was a brief discussion of some fascinating fossils of first rate importance. Except for stromatolites, they are one of only a handful of fossil organisms from a gigayear or more ago. Named *Bicellum* *brasieri,* it possesses two distinct cell types, and AFAIK this sets it apart from all fossil species from more than 600 million years ago.
> >>>
> >>> It was described scientifically here:
> >>>
> >>> "A possible billion-year-old holozoan with differentiated multicellularity,"
> >>> by Paul K. Strother, Martin D. Brasier, David Wacey, Leslie Timpe,
> >>> Martin Saunders, Charles H. Wellman,
> >>> Current Biology 31, 1--8 June 21, 2021
> >>>
> >>>
https://www.cell.com/action/showPdf?pii=S0960-9822%2821%2900424-3
> >>>
> >>> The article is free access. It includes many detailed photographs of a high magnification.
> >>>
> >>> The title is guardedly optimistic about it being an animal. It does try to narrow it down somewhat:
> >>>
> >>> "It seems reasonable to assume that Bicellum falls within one of the lineages leading
> >>> to one of the six clades that possess complex multicellularity today: animals, plants, florideophyte algae,
> >>> brown algae, ascomycete fungi, and basidiomycete fungi."
> >>>
> >>> The words "of eukaryotes" should have been inserted after "six clades".
> >
> >> "complex" would seem to rule out any of the various multicellular
> >> prokaryotes.
> >
> > It also rules out these cells. The quoted "complex multicellularity" is pure gravy.
> If that's pure gravy, how much more gravious would addding "eukaryote" be?
It would have made the focus sharper on what they were talking about, as well as
helping to understand how "complex multicellularity" is defined by the authors.
Their definition is demanding enough to exclude a seventh and eighth clade of eukaryotes: the plasmodial slime molds
and the cellular slime molds. One plasmodial slime mold produces 8 different kinds of reproductive cells:
When *Physarum* *polycephalum* is ready to make its reproductive cells, it grows a bulbous extension of its body to contain them.[15] Each cell is created with a random combination of the genes that the slime mold contains within its genome. Therefore, it can create cells with up to eight different gene types. Once these cells are released, they are independent and tasked with finding another cell it is able to fuse with.
https://en.wikipedia.org/wiki/Slime_mold
Then this webpage talks about a cellular slime mold that produces eleven different gene types.
Before we get further, it may
> > By the way, I saw no argument in the paper that they ought to be on the stem of one of the six clades; why not
> > a clade that includes two or three of them, eh?
> Because multicellularity arose independently within each of those six
> clades and can't parsimoniously be attributed to the common ancestor of
> any two of them.
As you know, characters can frequently be lost, and multicellularity strikes me as being one of them,
as long as some cell is capable of reproducing on its own. Some slime molds may be like that.
> > Remember all the different ways of rooting the tree Eukaria, that could not be ruled out? We took a
> > good long look at them on the talk.origins thread where the mysteries of meiosis were the topic.
> Yes, and none of those ways changes the number of independent origins of
> multicellularity.
That may come down to how you define "multicellularity." Cellular slime molds are unicellular
for most of their life cycles. As for plasmodial slime molds, they could either be regarded in
the main part of their life cycles as one giant cell with innumerable nuclei, or a whole lot of
cells that do not have partitions between the individual cells.
> >>> They do try to rule out prokaryotes with:
> >>>
> >>> "... these cells probably lacked rigid cell walls. This eliminates both cyanobacteria and the eukaryotic chlorophyte algae as likely homologs, because multicellular form in these taxa is strongly influenced by their
> >>> possession of rigid cell walls."
> >>>
> >>> This reliance on a single feature of *extant* cyanobacteria (some of which do have two very distinct kinds of cells)
> >>> is a weakness which calls for independent corroboration, provided by more characteristically
> >>> eukaryotic features.
> >
> >
> >> How big are these cells?
> >
> > Haven't you looked at the paper yet? They are quite small: the whole organism is less than 50 microns in diameter,
> > and the individual cells less than 5 microns.
> That seems quite small for a eukaryotic cell, doesn't it? It's large for
> a prokaryote, but not unreasonably large. The average size is 2.5
> microns.
That's true of the ones in the inner cell masses of *Bicellum* *brasieri* Of the external cells, they say the following:
"The elongate cells that form the peripheral layer are around 1.5 to 2 [microns] in diameter and generally about 3 to 4 times that in length, although, in some cases, they can be much longer (e.g., Figure 2K). The average width-to-length ratio for a set of 6 specimens was 0.28 (Table S1)."
My earlier "less than 5 microns" was due my relying on the photographs which showed the elongate cells
in cross section.