Monday, 16 December 2013

"20 scientific facts seldom taught to students" critically reviewed #10 "There is no evidence for the evolution of multicellularity"

Collyer’s tenth claim was that “while single-celled creatures are numerous, there are none with two, three, four, or even twenty cells. Thus there is no evolutionary sequence from single-celled to multi-celled creatures.”

In short, we have:

  • Argument from personal incredulity
  • Ignorance of the fact thatone would not reasonably expect tiny, soft-bodied multicellular life to be found in the fossil record. (We do have some evidence however which allows us to make some inferences)
  • Failure to acknowledge that single celled animals can be induced to form multicellular forms of life in the laboratory.
Apart from being wrong, it is also confusingly written. Is Collyer claiming there are no multicellular fossils, or that no such animals exist? Either way, he’s wrong. Molecular and cell biologists do not regard the evolution of multicellularity as intractable. The formation of colonies is regarded as the leading hypothesis:

It seems likely that an early step in the evolution of multicellular organisms was the association of unicellular organisms to form colonies. The simplest way of achieving this is for daughter cells to remain together after each cell division. Even some procaryotic cells show such social behavior in a primitive form. Myxobacteria, for example, live in the soil and feed on insoluble organic molecules that they break down by secreting degradative enzymes. They stay together in loose colonies in which the digestive enzymes secreted by individual cells are pooled, thus increasing the efficiency of feeding (the "wolf-pack" effect). These cells indeed represent a peak of social sophistication among procaryotes, for when food supplies are exhausted, the cells aggregate tightly together and form a multicellular fruiting body, within which the bacteria differentiate into spores that can survive even in extremely hostile conditions. When conditions are more favorable, the spores in a fruiting body germinate to produce a new swarm of bacteria.

Green algae (not to be confused with the procaryotic "blue-green algae" or cyanobacteria) are eucaryotes that exist as unicellular, colonial, or multicellular forms. Different species of green algae can be arranged in order of complexity, illustrating the kind of progression that probably occurred in the evolution of higher plants and animals. Unicellular green algae, such as
Chlamydomonas, resemble flagellated protozoa except that they possess chloroplasts, which enable them to carry out photosynthesis. In closely related genera, groups of flagellated cells live in colonies held together by a matrix of extracellular molecules secreted by the cells themselves. The simplest species (those of the genus Gonium) have the form of a concave disc made of 4, 8, 16, or 32 cells. Their flagella beat independently, but since they are all oriented in the same direction, they are able to propel the colony through the water. Each cell is equivalent to every other, and each can divide to give rise to an entirely new colony. Larger colonies are found in other genera, the most spectacular being Volvox, some of whose species have as many as 50,000 or more cells linked together to form a hollow sphere. In Volvox the individual cells forming a colony are connected by fine cytoplasmic bridges so that the beating of their flagella is coordinated to propel the entire colony along like a rolling ball. Within the Volvox colony there is some division of labor among cells, with a small number of cells being specialized for reproduction and serving as precursors of new colonies. The other cells are so dependent on one another that they cannot live in isolation, and the organism dies if the colony is disrupted.[1]

Unicellular life abounds, as any microbiologist will attest, which leads to the not unreasonable question of why would multicellular life evolve if a very good living can be made as a single celled organism. A number of arguments have been postulated:

Theoretical work suggests that a multicellular existence could have been advantageous by reducing predation, improving the efficiency of food consumption, facilitating more effective means of dispersal, limiting interactions with noncooperative individuals, or dividing labor. For example, unicellular lifestyle conflicts, such as the dependence of flagellum-induced motility and mitosis on the same molecular machinery, or the requirement for spatial or temporal separation of certain metabolic processes, could have been easily resolved in a multicellular setting by functional specialization, at least in principle.[2]

Furthermore, some of these theoretical ideas have been tested experimentally, confirming that there is indeed a non-trivial benefit in evolving multicellularity:

In several instances, theoretical expectations have been put to the test. The results have demonstrated that several reasons typically associated with transitions to multicellularity, such as predation avoidance or higher feeding efficiency, do indeed confer a selective advantage over unicellularity. For example, a number of algal species were able to evolve multicellularity when grown in culture in the presence of predators, thus dramatically reducing their chances of being eaten. Similarly, Volvox algae and myxobacteria have been shown to be at advantage when multicellular because of their ability to better utilize available nutrients. [3]

Recently, William Ratcliff et al demonstrated the experimental evolution of multicellularity in the unicellular yeast Saccharomyces cerevisiae by placing yeast in an environment where multicellularity would be expected to convey some advantage, and found that multicellularity rapidly evolved. The authors note:

Although known transitions to complex multicellularity, with clearly differentiated cell types, occurred over millions of years…we have shown that the first crucial steps in the transition from unicellularity to multicellularity can evolve remarkably quickly under appropriate selective conditions. Multicelled snowflake-phenotype yeast evolved in all 15 replicate populations, in two separate experiments, within 60 d of settling selection. All snow-flake yeast formed clusters through postdivision adhesion, not aggregation. This method of growth ensures high relatedness among individual cells within the cluster, aligning the fitness of individual cells with their genetically identical kin within a cluster.

Aggregation through floc-type adhesion was not observed in any of our experiments, possibly because this method of growth is prone to within-group conflict). Choanoflagellates, the closest unicellular ancestor to animals, can form multicellular colonies through postdivision adhesion), raising the possibility that a similar step was instrumental in the evolution of animal multicellularity.

We observed adaptation of multicellular traits, indicating a shift in selection from individual cells to multicellular individuals. In response to selection for even more rapid settling, snowflake-phenotype yeast adapted through changes in their multicellular life history, increasing the length of the juvenile phase that pre-cedes production of multicellular propagules. We also observed the evolution of division of labor within the cluster: most cells remain viable and reproduce, but a minority of cells become apoptotic. Apoptotic cells act as break points within multicellular clusters, allowing snowflake yeast to produce a greater number of propagules from a given number of cells. This is functionally analogous to germ-soma differentiation, where cells specialize into reproductive and nonreproductive task. These results demonstrate that multicellular traits readily evolve as a consequence of among-cluster selection.[4]

Suffice it to say that Collyer’s claim that “there is no evolutionary sequence from single-celled to multi-celled creatures” has been experimentally invalidated by these experiments showing the evolution of multicellularity in previously single-celled organisms.

Special creationist attacks on evolution often focus on the fossil record. One common assertion is that the fossil record is incomplete and does not support evolution. The latter claim is fallacious, as we do have evidence of large-scale evolutionary change and abundant transitional fossils.[5] The other attack argues that the fossil record must contain a continuous sequence of gradual change, and any deviation from this invalidates evolution. This claim betrays a deep ignorance of taphonomy, the science of how dead animals decay and fossilise. Not every environment is amenable to preserving fossils. Marine environments are better at preserving life than terrestrial. Forests are some of the worst environments as their acidic soil dissolves bones before they can fossilise. Hard bodied animals fossilise more easily than soft bodied animals. Furthermore, most dead animals are consumed by scavengers before they can fossilise. The fact we have a fossil record at all is remarkable.

Fossil evidence of early multicellular life does exist[6] and it does provide assistance in reconstructing the early history of life. Two examples include:

Shuiyousphaeridium macroreticulatum, which was a species of microfossil found in the coastalmarine shales of the Ruyang Group in northern China. It has been dated to no younger than 1.25 billion years, and is regarded as a eukaryote, with possible classifications ranging from fungus to dinoflagellate.[7]

Tappania plana[8] is another eukaryote fossil, dated to around 1.5 billion years ago. As one would imagine, ancient microfossils would pose some taxonomic challenges, but the authors state that it was a eukaryote with a complex cytoskeleton, and possibly heterotrophic (organisms such as fungi, animals and some bacteria) that are not capable of creating their own food sources.

Shuiyousphaeridium macroreticulatum from the Mesoproterozoic Ruyang Group, China. (a) Light microphotograph showing specimen with numerous regularly spaced cylindrical processes that flare outward; (b, c, e–f) SEM images showing (b) whole specimen, with inset showing details of process morphology, (c) outer wall surface covered with ridges that delimit granular polygonal fields, (e) wall reticulation and (f) inner wall surface of closely packed, beveled hexagonal plates; (d) TEM image showing the two appressed walls of a single specimen—note multilayered wall comprising a thick electron-dense homogeneous layer of organic plates (ii) between an outer layer of debris and processes—note base of process at bottom left of centre (iii) and a thin electron-tenuous layer (i) that lines the inner side of plates. Scale bar in a=57 μm for a, 50 μm for b (20 μm for inset), 1.2 μm for c and e, 0.5 μm for d and 2.5 μm for f. Philosophical Transactions- Royal Society of London Series B Biological Sciences 361(1470): 1023-1038.
The authors note that:

In summary, late Paleoproterozoic and early Mesoproterozoic rocks preserve evidence for a moderate diversity of preservable eukaryotic organisms. This evidence includes cell walls without surface ornament (but with complex ultrastructure) and walls with regularly distributed surface ornamentation, with asymmetrically arranged processes that appear to reflect active cell growth, and with numerous symmetrically arranged processes. Collectively, these fossils suggest that eukaryotes not only existed in mid-Proterozoic oceans, but possessed flexible membranes and cytoskeletons capable of directing cell remodeling and surface morphology.[9]

 Diversity of Late Palaeoproterozoic to Early Mesoproterozoic eukaryotic fossils. (a) Tappania plana, from the Early Mesoproterozoic Roper Group, Australia; (b) Horodyskia moniliformis, from the Mesoproterozoic Bangemall Group, Western Australia; (c,f) Satka favosa, from the Roper Group, (c) showing the wall construction of hexagonal plates, shown under SEM in (f); (d, e) Valeria lophopstriata, showing ornamentation of closely spaced parallel ridges on the inner wall surface in SEM (d) and light microscopic (e) view; (g, h) Leiosphaeridia sp., an unornamented spheroidal acritarch, with a complex wall composed of two electron-dense, homogeneous layers (i) that sandwich a thick central layer with electron-dense, porous texture (ii) visible in TEM cross-section (h); Grypania spiralis, a coiled macrofossil compression from the Mesoproterozoic Gaoyuzhuang Formation, China (courtesy of M. Walter). Scale bar=40 μm for (a), 7.8 mm for (b), 35 μm for (c), 4 μm for (d), 15 μm for (e), 7.5 μm for f, 1 μm for (h), and 3 mm for (i). - Philosophical Transactions- Royal Society of London Series B Biological Sciences 361(1470): 1023-1038.
 Of course, the question many ask is whether we are able to place these fossils into an evolutionary family tree. This is difficult at best due to the age and size of the fossils. The task is not completely impossible however:

By 750-800 Ma, the most diverse fossil assemblages contain an increased diversity of acritarch and other protistan morphotypes, including small branched structures interpreted as siphonocladalean green algae; vase-shaped structures interpreted as both filose and lobose testate amoebae – and, therefore, as both amoebozoan and cercozoan protists; remarkable spheroidal fossils from which numerous anastomosing cellular filaments arise, interpreted as possible fungi; and a moderate diversity of other colonial to multicellular eukaryotes with less certain systematic affinities. Collectively, carefully studied microfossil assemblages support the hypothesis that the later Mesoproterozoic and early Neoproterozoic was a time of major clade divergence within the Eucarya, although diversity within most major clades remained relatively low.[10]  (Emphasis mine)

A functional approach to classification of the fossils can be taken, as the authors note, and this can allow one to avoid some of the problems imposed on examining the evidence by difficulties in placing them in taxonomic groups:

As summarized in the preceding section, Proterozoic protists included simple, unornamented unicells; morphologically complex and elaborately ornamented vesicles; clusters (colonies?) of uniform cells; uniseriate filaments, both branched and unbranched; and three-dimensionally complex multicellular organisms displaying cellular differentiation. For many of these fossils, phylogenetic placement is difficult because few characters ally fossil populations with extant clades. We can sidestep these issues by focusing on the characters themselves, and not on any implied phylogenetic affinity. For example, Tappania may or may not be related to fungi, but it inarguably displays an asymmetrical arrangement of processes. Similarly, one might debate the attribution of Bangiomorpha to the red algae, but it demonstrably exhibits cell differentiation. This suggests an independent, “taxonomy-free” avenue of inquiry focused on function, development, and the cell biological processes that underlie these features.[11] 

Late Mesoproterozoic and Neoproterozoic eukaryotic fossils: (a, b) ‘Tappania plana’ from the Neoproterozoic Wynniatt Formation, arctic Canada, a complex form with septate, anastomosing processes, shown in detail in (b); (c) Bangiomorpha pubescens, from the Late Mesoproterozoic Hunting Formation, arctic Canada, showing radial division of cells within a discrete zone of uniseriate filaments; (d) Konglingiphyton erecta, a macroscopic, dichotomously branched alga from the Ediacaran Doushantou Formation, China; (e) Eosaccharomyces ramosa from the Late Mesoproterozoic Lakhanda succession, Siberia, showing net-like distribution on a bedding surface, with cells aligned along strands; (f) Segmentothallus asperus from the Lakhanda succession, a large uniseriate filament; (g) Appendisphaera grandis, a large acritarch with numerous, symmetrically arranged processes, from the Ediacaran Khamaka Formation, Siberia; (h) Kildinosphaera verrucata, an ornamented acritarch from the Neoproterozoic Miroyedikha Formation, Siberia; (i) Bonniea dacruchares, a vase-shaped protistan test from the Neoproterozoic Kwagunt Formation, Grand Canyon, USA; (j) preserved cast and mould of vase-shaped protist in silicified carbonates of the Neoproterozoic Ryssö Formation, Svalbard. Scale bar=100 μm in (a), 12 μm in (b), 40 μm in (c) 4 mm in (d) 150 μm in (e), 500 μm in (f), 70 μm in (g), 25 μm in (h), 43 μm in (i), and 75 μm in (j). - Phil. Trans. R. Soc. B 29 June 2006 vol. 361 no. 1470 1023-1038
We are likely never able to be in a position to plot with mathematical precision the pathway taken from single celled life to multicelluar life, but it demonstrates a gross ignorance of the literature to claim that we are completely in the dark. Apart from the fossil data, we have evidence from molecular genetics and experimental biology which allow us to construct relatively robust scenarios on the evolution of multicellular life.

It is also worth pointing out that the evolution of multicellularity is very much taught to students, as websites such as this one clearly demonstrate.

[1] Alberts B, Bray D, Lewis J, et al. Molecular Biology of the Cell. 3rd edition (New York: Garland Science; 1994)

[2] Rokas A “The Origins of Multicellularity and the Early History of the Genetic Toolkit for Animal Development” Annu. Rev. Genet. 2008. 42:235–51

[3] ibid, p 137

[4] Ratcliff WC,  Denison RF, Borrello M, Travisano M  Experimental evolution of multicellularity. Proc Natl Acad Sci USA (2012) 109:1595–1600

[5] Prothero DR “Evolution: What the Fossils Say and Why It Matters” (2007, Columbia University Press)

[6] Knoll, Andrew H., Emmanuelle J. Javaux, David Hewitt, and Phoebe A. Cohen. 2006. Eukaryotic organisms in Proterozoic oceans. Philosophical Transactions- Royal Society of London Series B Biological Sciences 361(1470): 1023-1038.

[7] ibid, p 5

[8] ibid, p 6

[9] ibid, p 11

[10] ibid, p 13

[11] ibid, p 19