Discredited hypotheses for the Cambrian explosion

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As understanding of the events of the Cambrian becomes clearer, data have accumulated to make some postulated causes for the Cambrian explosion look improbable. Some examples are the evolution of herbivory, vast changes in plate tectonic rates or orbital motion, or different evolutionary mechanisms in force.

Contents

Developmental explanations

Regulatory genes

Hox genes regulate the operation of other genes by switching them on or off in various parts of the body, for example “make an eye here” or “make a leg there”. Very similar Hox genes are found in all animals from Cnidaria (e.g. jellyfish) to humans, although mammals have four sets of Hox genes while Cnidaria have only one. [1] Hox genes in different animal groups are so similar that, for example, one can transplant a human “make an eye” Pax6 gene into a fruitfly embryo and it still causes an eye to form – but it is a fruitfly eye, because the genes that the transplanted Hox gene activates are fruitfly genes. [2]

The fact that all animals have such similar Hox genes strongly suggests that the last common ancestor of all bilaterians had similar Hox genes. This led to speculation that the evolution of Hox genes may have paved the way for the development of complex body forms.

This is where molecular dating can be useful. A rough estimate of the age of a lineage, and indeed genes, can be achieved by assuming that mutations accumulate at a constant rate in the genome. While the dates produced by this technique are often imprecise, and vary from method to method, the technique can provide an indication of the 'relative' age of appearances.

Molecular clocks detect the radiation of three major clades of bilaterians – lophotrochozoans, deuterostomes, and ecdysozoans – during the early Cambrian. The same study also found that Hox genes diversified before these groups did – meaning that they could not have been the innovation that caused the explosion. [3]

Developmental entrenchment

Several scientists suggest that, as organisms become more complex, changes to their basic developmental mechanisms are more likely to be disadvantageous. In this way, the basic body design becomes "stuck in a rut", because any mutation that affects such central mechanisms will probably prevent the organism from developing; mutations are only likely to be advantageous where they affect less fundamental parts of the organism. [4] By analogy, it is difficult to make modifications to the foundations of a building without causing it to collapse, whereas alterations can easily be made to the upper floors. The taller the building grows, the more likely a modification to the foundations will be to cause it to collapse.

However, as understanding of the developmental pathways of organisms has improved, even the original proponent of this idea has realised that it is not likely to be a major factor, [5] and examples of major post-Cambrian changes at a fundamental level have been found. [6]

Ecological explanations

The appearance of herbivorous organisms

Awramik (1973) suggested that the appearance about 700 million years ago of protists (single-celled eukaryotes) that "cropped" microbial mats greatly expanded food chains, and thus allowed rapid diversification, which led to the Cambrian explosion, [7] but "cropping" is now thought to have arisen before 1 billion years ago, as stromatolites began to decline about 1.25 billion years ago. [8] [9]

Environmental explanations

Methane release

Others have suggested that each short-term decrease in the 13C/12C ratio throughout the early Cambrian represents a methane “burp” which, by raising global temperatures, triggered an increase in diversity. [10] But this hypothesis also fails to explain the increase in disparity. [6]

Related Research Articles

Cnidaria Aquatic animal phylum having cnydocytes

Cnidaria is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments: they are predominantly marine.

Fossil Preserved remains or traces of organisms from a past geological age

A fossil is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in amber, hair, petrified wood, oil, coal, and DNA remnants. The totality of fossils is known as the fossil record.

Homeobox

A homeobox is a DNA sequence, around 180 base pairs long, found within genes that are involved in the regulation of patterns of anatomical development (morphogenesis) in animals, fungi, plants, and numerous single cell eukaryotes. Homeobox genes encode homeodomain protein products that are transcription factors sharing a characteristic protein fold structure that binds DNA to regulate expression of target genes. Homeodomain proteins regulate gene expression and cell differentiation during early embryonic development, thus mutations in homeobox genes can cause developmental disorders.

Bilateria All animals having a bilateral symmetry as embryo

The bilateria or bilaterians are animals with bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. This also means they have a head and a tail as well as a belly and a back. Nearly all are bilaterally symmetrical as adults as well; the most notable exception is the echinoderms, which achieve secondary pentaradial symmetry as adults, but are bilaterally symmetrical during embryonic development.

Stromatolite Layered sedimentary structures formed by the growth of bacteria or algae

Stromatolites or stromatoliths are layered mounds, columns, and sheet-like sedimentary rocks that were originally formed by the growth of layer upon layer of cyanobacteria, a single-celled photosynthesizing microbe. Fossilized stromatolites provide records of ancient life on Earth. Lichen stromatolites are a proposed mechanism of formation of some kinds of layered rock structure that are formed above water, where rock meets air, by repeated colonization of the rock by endolithic lichens.

Eumetazoa A basal animal clade as a sister group of the [[Sponge|Porifera]]

Eumetazoa or Diploblasts, or Epitheliozoa, or Histozoa are a proposed basal animal clade as a sister group of the Porifera. The basal Eumetazoan clades are the Ctenophora and the ParaHoxozoa. Placozoa is now also seen as a Eumetazoan in the Parahoxozoa.

Nerve net simplest form of a nervous system: neurons are found spread apart, allowing the organism to respond to its environment

A nerve net consists of interconnected neurons lacking a brain or any form of cephalization. While organisms with bilateral body symmetry are normally associated with a central nervous system, organisms with radial symmetry are associated with nerve nets. Nerve nets can be found in members of the Cnidaria, Ctenophora, and Echinodermata phyla, all of which are found in marine environments. Nerve nets can provide animals with the ability to sense objects through the use of the sensory neurons within the nerve net.

<i>Kimberella</i> Genus of molluscs

Kimberella is an extinct genus of bilaterian known only from rocks of the Ediacaran period. The slug-like organism fed by scratching the microbial surface on which it dwelt in a manner similar to the gastropods, although its affinity with this group is contentious.

Hox genes, a subset of homeobox genes, are a group of related genes that specify regions of the body plan of an embryo along the head-tail axis of animals. Hox proteins encode and specify the characteristics of 'position', ensuring that the correct structures form in the correct places of the body. For example, Hox genes in insects specify which appendages form on a segment, and Hox genes in vertebrates specify the types and shape of vertebrae that will form. In segmented animals, Hox proteins thus confer segmental or positional identity, but do not form the actual segments themselves.

Ediacaran biota Enigmatic tubular and frond-shaped, mostly sessile organisms that lived during the Ediacaran Period (ca. 635–542 Mya)

The Ediacaranbiota is a taxonomic period classification that consists of all life forms that were present on Earth during the Ediacaran Period. These were composed of enigmatic tubular and frond-shaped, mostly sessile, organisms. Trace fossils of these organisms have been found worldwide, and represent the earliest known complex multicellular organisms.

Animal Kingdom of motile multicellular eukaryotic heterotrophic organisms

Animals are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 millionths of a metre to 33.6 metres (110 ft). They have complex interactions with each other and their environments, forming intricate food webs. The kingdom Animalia includes humans, but in colloquial use, the term animal often refers only to non-human animals. The scientific study of animals is known as zoology.

Marine invertebrates

Marine invertebrates are the invertebrates that live in marine habitats. Invertebrate is a blanket term that includes all animals apart from the vertebrate members of the chordate phylum. Invertebrates lack a vertebral column, and some have evolved a shell or a hard exoskeleton. As on land and in the air, marine invertebrates have a large variety of body plans, and have been categorised into over 30 phyla. They make up most of the macroscopic life in the oceans.

Cambrian substrate revolution

The "Cambrian substrate revolution" or "Agronomic revolution", evidenced in trace fossils, is the diversification of animal burrowing during the early Cambrian period.

The Cambrian explosion or Cambrian radiation was an event approximately 541 million years ago in the Cambrian period when practically all major animal phyla started appearing in the fossil record. It lasted for about 13 – 25 million years and resulted in the divergence of most modern metazoan phyla. The event was accompanied by major diversification of other organisms.

The urbilaterian is the hypothetical last common ancestor of the bilaterian clade, i.e., all animals having a bilateral symmetry.

The origin of the brachiopods is uncertain; they either arose from reduction of a multi-plated tubular organism, or from the folding of a slug-like organism with a protective shell on either end. Since their Cambrian origin, the phylum rose to a Palaeozoic dominance, but dwindled during the Mesozoic.

Avalon explosion Proposed evolutionary event in the history of metazoaproducing the Ediacaran biota

The Avalon explosion, named from the Precambrian fauna of the Avalon Peninsula, is a proposed evolutionary radiation in the history of the Animalia, about 575 million years ago, some 33 million years earlier than the Cambrian explosion.

Stanley Awramik is an American biogeologist and paleontologist. He is best known for his work related to the Precambrian. In 2013, he was inducted as a fellow of the Geological Society of America.

The ParaHoxozoa are a proposed basal Diploblast/Eumetazoa clade as sister of the Ctenophora. It consists of the Triploblasts/Bilateria as well as the Placozoa and Cnidaria.

Evo-devo gene toolkit

The evo-devo gene toolkit is the small subset of genes in an organism's genome whose products control the organism's embryonic development. Toolkit genes are central to the synthesis of molecular genetics, palaeontology, evolution and developmental biology in the science of evolutionary developmental biology (evo-devo).

References

  1. ↑ Erwin, D.H.; Davidson, E.H. (July 1, 2002). "The last common bilaterian ancestor". Development (abstract). The Company of Biologists Limited. 129 (13): 3021–3032. PMID   12070079.
  2. ↑ "Understanding complexity". University of California, Berkeley.
  3. ↑ de Rosa; Grenier, JK; Andreeva, T; Cook, CE; Adoutte, A; Akam, M; Carroll, SB; Balavoine, G (1999). "Hox genes in brachiopods and priapulids and protostome evolution". Nature. 399 (6738): 772–6. Bibcode:1999Natur.399..772D. doi:10.1038/21631. PMID   10391241.
  4. ↑ Valentine, J.W. (1986). "Fossil record of the origin of Baupläne and its implications". In Raup, D.M.; Jablonski, D. (eds.). Patterns and Processes in the History of Life. pp. 209–231.
  5. ↑ Valentine, J.W. (April 1995). "Why No New Phyla after the Cambrian? Genome and Ecospace Hypotheses Revisited". PALAIOS (abstract). SEPM Society for Sedimentary Geology. 10 (2): 190–194. Bibcode:1995Palai..10..190V. doi:10.2307/3515182. JSTOR   3515182.
  6. 1 2 Marshall, C.R. (2006). "Explaining the Cambrian "Explosion" of Animals". Annual Review of Earth and Planetary Sciences (abstract). 34 (1): 355–384. Bibcode:2006AREPS..34..355M. doi:10.1146/annurev.earth.33.031504.103001. S2CID   85623607.
  7. ↑ Stanley, S.M. (May 1973). "An Ecological Theory for the Sudden Origin of Multicellular Life in the Late Precambrian". Proc Natl Acad Sci USA. 70 (5): 1486–1489. Bibcode:1973PNAS...70.1486S. doi:10.1073/pnas.70.5.1486. PMC   433525 . PMID   16592084.
  8. ↑ McNamara, K.J. (20 December 1996). "Dating the Origin of Animals". Science. 274 (5295): 1993–1997. Bibcode:1996Sci...274.1993M. doi: 10.1126/science.274.5295.1993f .
  9. ↑ Awramik, S.M. (19 November 1971). "Precambrian columnar stromatolite diversity: Reflection of metazoan appearance". Science (abstract). 174 (4011): 825–827. Bibcode:1971Sci...174..825A. doi:10.1126/science.174.4011.825. PMID   17759393 . Retrieved 2007-12-01.
  10. ↑ Kirschvink, J.L.; Raub, T.D. (January 2003). "A methane fuse for the Cambrian explosion: carbon cycles and true polar wander" (PDF). Comptes Rendus Geoscience. 335 (1): 65–78. Bibcode:2003CRGeo.335...65K. doi:10.1016/S1631-0713(03)00011-7.
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