The ancestors of comb jellies such as Mnemiopsis leidyi may be the earliest creatures in the animal kingdom.
Image: William Browne/Univ. of Miami
-
Why Is the Penis Shaped Like That?
Why do testicles hang the way they do? Is there an adaptive function to the female orgasm? What does it feel like to want to kill yourself? Does “free will”...
Read More »
Animals evolved gradually, from the lowly sponge to the menagerie of tentacled, winged and brainy creatures that inhabit Earth today. This idea makes such intuitive sense that biologists are now stunned by genome-sequencing data suggesting that the sponges were preceded by complex marine predators called comb jellies.
Although they are gelatinous like jellyfish, comb jellies form their own phylum, known as ctenophores. Trees of life typically root the comb jellies' lineage between the group containing jellyfish and sea anemones and the one containing animals with heads and rears — which include slugs, flies and humans. Comb jellies paddle through the sea with iridescent cilia and snare prey with sticky tentacles. They are much more complex than sponges — they have nerves, muscles, tissue layers and light sensors, all of which the sponges lack.
“It’s just wild to imagine” that comb jellies evolved before sponges, says Billie Swalla, a developmental biologist at the University of Washington in Seattle and a leading member of the team sequencing the genome of the comb jelly Pleurobrachia bachei. But the team is suggesting just that, in results they presented at the annual meeting of the Society for Integrative and Comparative Biology, held on 3–7 January in San Francisco, California.
Despite comb jellies' complexity, DNA sequences in the Pleurobrachia genome place them at the base of the animal tree of life, announced Swalla's colleague Leonid Moroz, a neurobiologist at the University of Florida in Gainesville. Another team presented results from genome sequencing for the comb jelly Mnemiopsis leidyi, and found that the phylum lands either below, or as close to the base as, sponges on the tree.
“We’ve always thought that predator–prey interactions and sensory adaptations evolved long after the origin of sponges,” Swalla says. “Now we need to imagine early life as a sponge, ctenophore and everything in between.” Because millions of species have gone extinct since animals appeared some 542 million years ago, Swalla says, the ancestor of all animals might look different from modern comb jellies and sponges.
Gene families, cell-signaling networks and patterns of gene expression in comb jellies support ancient origins as well. For example, Moroz and his team found that comb jellies grow their nerves with unique sets of genes. “These are aliens,” Moroz jokes. He suggests that comb jellies might be descendants of Ediacaran organisms, mysterious organisms that appear in the fossil record before animals. Indeed, in 2011, paleontologists claimed that one of these 580-million-year-old fossils resembled comb jellies.
Andy Baxevanis, a comparative biologist at the US National Human Genome Research Institute in Bethesda, Maryland, and a leader on the Mnemiopsis genome project, says that comb jellies are the only animals that lack certain genes crucial to producing microRNA — short RNA chains that help to regulate gene expression. Moreover, he points out, sponges and comb jellies lack other gene families that all other animals possess.
If comb jellies evolved before sponges, the sponges probably lost some of their ancestors' complexity. Alternatively, says Sally Leys, a biologist at the University of Alberta in Edmonton, sponges may have complexity that scientists have yet to appreciate. “A lot of sponges are more exquisite than a lump on a rock,” she says.
Skeptics wonder whether a high rate of genetic mutation in comb jellies might be causing the lineage to seem closer to the bottom of the tree than it really is. “In the analyses I’ve done, ctenophores are the most problematic taxon. They jump around depending on which genes you use and which animals you include,” says Gert Wörheide, a molecular paleobiologist at the Ludwig Maximilian University of Munich in Germany. At the meeting, Wörheide presented a tree of life created by comparing ribosomal protein sequences. In it, sponges remained rooted in their bottom-most spot.
This article is reproduced with permission from the magazine Nature. The article was first published on January 8, 2013.
See what we're tweeting about
7 Comments
Add CommentRe: "...Leonid Moroz, a neurobiologist...suggests that comb jellies might be descendants of Ediacaran organisms, mysterious organisms that appear in the fossil record before animals."
Reply | Report Abuse | Link to thisThat would seem to raise the question: From what did the Ediacaran organisms evolve?
Re: "If comb jellies evolved before sponges, the sponges probably lost some of their ancestors' complexity."
That would seem to cover all the bases; "evolution" will sometimes result in reduced complexity, at other times, increased complexity. The late Prof. Stephen Jay Gould, in a rebuke of creationists, wrote: "Unbeatable systems are dogma, not science."
[Evolution as Fact and Theory, online edition, para 15,
http://www.stephenjaygould.org/library/gould_fact-and-theory.html]
OMG, Bill Crofut, Christian moron, if the world is only 6,000 years old, as you believe, where did 580-million-year-old fossils come from??? How "unbeatable" is it for you to refuse to answer that question???
Reply | Report Abuse | Link to thiswombat99,
Reply | Report Abuse | Link to thisRe: "...Bill Crofut, Christian moron..."
1:23 But we preach Christ crucified: unto the Jews indeed a stumblingblock, and unto the Gentiles foolishness:
4:10. We are fools for Christs sake...
THE FIRST EPISTLE OF ST. PAUL TO THE CORINTHIANS
[Catholic Bible. (c) 2000. Douay Rheims translation. Murray, KY: A production of Catholic Software]
Re: "...where did 580-million-year-old fossils come from???"
Perhaps they were conjured up in the evolutionary imagination:
C. Patterson, G. Tilton and M. Inghram authored the seminal paper which catapulted us from,
“...the generally accepted estimate of 3.3 x 10^9 yr...” to “...about 4.5 x 10^9 yr...”
[1955. Age of the Earth. SCIENCE, 21 January, p. 69]
The paper is replete with assumptions and the following warning which would seem to have gone unheeded:
“It should be recognized that an approximate age value is sufficient and should be viewed with considerable skepticism until the basic assumptions that are involved in the method of calculations are verified” (p. 75).
Where, in the scientific literature will one find verification of the basic assumptions?
To Bill Crohut:
Reply | Report Abuse | Link to thisWhere, in the religious literature for that matter, will one find verification of the basic assumptions of biological ages of living beings?
Why not sponge it over...
A standard fundamentalist tactic is latch onto details subject to revision as knowledge progresses, while ignoring the totally certain major issues. It is totally certain, from multiple methods, that the Earth's rocks are billions of years old, versus 6,000 according to the Bible, so why latch onto the exact figure -- that's just a detail subject to revision. And it is totally certain that those rocks contain fossils of extinct creatures unmentioned in the Bible, but in a sequence from microorganisms to marine invertebrates to primitive land plants and amphibians to modern mammals, etc., that shows the earlier fossils were the ancestors of the later ones -- and of those organisms alive today. So evolution is a fact -- it's only Darwin's proposed mechanism that may be called a theory that attempts to explain how the fact of evolution took place. If Bill Crofut disproved Darwin's theory of natural selection tomorrow, evolution would remain a fact, proven by fossils, that would require some other mechanism. His only possible out, I guess, is the absurd one proposed by a 19th-century U.S. Christian -- that God created the Earth as a "going concern," with fake fossils created in it!
Reply | Report Abuse | Link to thissunnystrobe,
Reply | Report Abuse | Link to thisHave you any specific religious literature in mind?
wombat99,
Reply | Report Abuse | Link to thisLet me begin by clearing up a misconception. As a Traditional Roman Catholic, the title, "fundamentalist," does not apply to me. You need not take my word for that; ask any fundamentalist.
Re: "...details subject to revision as knowledge progresses..."
Here's an example of progressive knowledge:
"These denials of modern geology are astonishing in the extreme to any scientist familiar with the depth and strength of the evidence for a 4.54 billion-year-old earth."
http://www.huffingtonpost.com/david-h-bailey/what-on-earth-do-they-thi_b_2162651.html
Imagine that! it only took nearly 60 years to "progress" from 4.5 to 4.54 billion years for the age of the Earth. Yet, there seems to be something missing:
Where, in the scientific literature will one find verification of the basic assumptions?
Re: "... fossils of extinct creatures unmentioned in the Bible..."
Fossilization is indirectly mentioned in the Bible: Genesis 6:5 through 8:14.
Re: "If Bill Crofut disproved Darwin's theory of natural selection tomorrow, evolution would remain a fact..."
It isn't necessary for me to disprove natural selection:
"The view that natural selection, leading to the survival of the fittest, in populations of individuals of varying characteristics and competing amongst themselves, has produced in the course of geological time gradual transformations leading from a simple primitive organism to the highest forms of life, without the intervention of any directive agency or force, is thus the essence of the Darwinian position....That natural selection directs the course of evolution Darwin could not prove by an appeal to facts... Darwin was inconsistent since, in his view, natural selection acts not only by the survival of the, fittest but also by the extermination of the less fit and may produce anatomical degradation as well as improvement."
[Prof. W. R. Thompson. 1956. Introduction. In: Charles Darwin. Origin of Species. Everyman Library No. 811. London: J. M. Dent and Sons. Reprinted with permission. Evolution Protest Movement. 1967. NEW CHALLENGING ‘INTRODUCTION' TO THE ORIGIN OF SPECIES. Selsey, Sussex: Selsey Press Ltd., p. 6, 7, 11]