[X] My Assistant

[Guest_Richard Trigaux_*]

The Call That Is Important To Us All

Leigh Dayton

Science Writer

Earthlings haven't yet heard from ET, but leading questers for cosmic company are getting ready to take the call... just in case.

They've established an international committee to set the etiquette for inter-galactic contact. And Sydney-based cosmologist Paul Davies just took on the top job.

"We need to get the protocol correct and clarified," noted Professor Davies, with Macquarie University's Australian Centre for Astrobiology.

"If ET called tonight we'd be in a bit of a muddle about it all," he added, speaking prior to his first meeting as head of the Post-Detection Science and Technology Taskgroup.

The body is part of the International Academy of Astronautics and was founded by radio-astronomer Jill Tarter of the Search for Extraterrestrial Intelligence (SETI) Institute in California.

"We want the correct details about the discovery event and any interpretation made about it to get out there," commented Dr Tarter, in Australia for a lecture tour supported by Sydney University's Centre for Human Aspects of Science and Technology.

According to Dr Tarter - whose SETI exploits were portrayed by Jodie Foster in the 1997 film Contact - if a signal from ET is detected it must be verified quickly and the news spread widely to ensure it's not "co-opted" by interest groups or politicians.

"Another caveat is that (scientists) will not transmit a reply until there's a global consensus about whether to reply and what should be said," Dr Tarter claimed.

The rest is here:

http://www.theaustralian.news.com.au/commo...5E29098,00.html

Interesting paper, ljk4-1.

The problem if "a signal is found" is twofold:
-avoid any mistake, and thus avoiding publishing before being sure
-avoid any censorship, and thus publishing as fast as possible.

This is contradictory, so it needs a special organization such as:
-contact many scientists under secrecy
-mobilize great science instruments in higher priority
-protect the evidences
-spread the information to the public by several different and numerous channels, especially internet sites in several countries

Hey hey SETI begins to look like a sci-fi story...

With my opinion, the first evidence of an extraterrestrial civ is very likely to be unintentionnal (from them). But if we find a probe orbiting around our sun...

The scenario in the "contact" movie is not realistic: if other civilizations reached a higher level, they are much more likely to have reached for long ago, not just in the same time than ours. So they are very likely to know our existence from very long too, not just discovering us when the first TV broadcast is send away. IF THERE ARE TECHNOLOGICAL EXTRATERRESTRIAL INTELLIGENCES, THEY KNOW OUR EXISTENCE. That they did not tried to actually contact us is a bit surprising. There is a wide range of very different explanations.

[tty]

Note that "severe effect on the biota" does not means that the evolution is stopped, on the countrary such events may speed it up, as with the Chixculub impact which allowed the supremacy of mammals, hot blooded animals which were able to develop intelligent brains, when cold-blooded reptilians could not.

That is a very simplified view. Mammals (synapsids may be a better word) are a very old group that was dominant before the dinosaurs and were outcompeted by them, persisting only in small-animal niches. The disappearance of the dinosaurs allowed them to diversify strongly, one of the results being us.
Also it is far from clear that dinosaurs were "cold-blooded", bird which are (highly derived) dinosaurs are rather more "hot-blooded" than mammals, and the difference between "cold-blooded" and "hot-blooded" animals was probably far from sharp back in the Cretaceous. As to whether dinosaurs could have become intelligen given another 65 million years of evolution, we will never know. Some late Cretaceous theropods had fairly large brains, and recent research suggest that the most intelligent birds (corvids and parrots) are about as smart as monkeys.
One thing is for sure. If that asteroid hadn't hit at Chicxulub 65 million years ago we wouldn't be here.

tty

[ljk4-1]

Interesting paper, ljk4-1.

The problem if "a signal is found" is twofold:
-avoid any mistake, and thus avoiding publishing before being sure
-avoid any censorship, and thus publishing as fast as possible.

This is contradictory, so it needs a special organization such as:
-contact many scientists under secrecy
-mobilize great science instruments in higher priority
-protect the evidences
-spread the information to the public by several different and numerous channels, especially internet sites in several countries

Hey hey SETI begins to look like a sci-fi story...

With my opinion, the first evidence of an extraterrestrial civ is very likely to be unintentionnal (from them). But if we find a probe orbiting around our sun...

The scenario in the "contact" movie is not realistic: if other civilizations reached a higher level, they are much more likely to have reached for long ago, not just in the same time than ours. So they are very likely to know our existence from very long too, not just discovering us when the first TV broadcast is send away. IF THERE ARE TECHNOLOGICAL EXTRATERRESTRIAL INTELLIGENCES, THEY KNOW OUR EXISTENCE.  That they did not tried to actually contact us is a bit surprising. There is a wide range of very different explanations.

The ETI in Contact were way ahead of us (see the novel especially). With 400 billion star systems to monitor, and we just started broadcasting about a century ago (in a galaxy that is at least 10 billion years old), it would not be that surprising if even an advanced ETI missed little old us.

I am also thinking of that first season episode of Star Trek: The Next Generation where an alien named the Traveler visits the Enterprise. When Riker asks why his kind never bothered to contact humans until now, his response was that we weren't interesting until then. I think there is a lot of truth to that statement. We might be of interest to an alien anthropologist, but what could we teach beings who can explore other stars with vessels?

And one more from Star Trek: The fourth film from 1986 had an ETI making contact with the most advanced species on Earth - and it wasn't us.

There is a series of images in the companion book and the film of Timothy Ferris' Life Beyond Earth that show the electromagnetic sphere surrounding our planet and spreading into the galaxy at the speed of light. As we move away from the sphere into deep space, it shows graphically how little we have really penetrated in this regard into the wider Universe.

http://www.pbs.org/lifebeyondearth/

So in addition to an advanced ETI probably having little reason to talk to or with us, we will not be obvious to most of the Milky Way for ages.

Of course you could argue that a species might examine every star with a reasonable chance of having planets with life, but since we now know that even red dwarfs might be friendly to life, that only adds to the time-consuming process.

But one thing I do agree with: If we detected an ETI signal now, humanity's reaction would be a muddled mess. But maybe it is just what we need.

[ljk4-1]

Paper: astro-ph/0512291

Date: Mon, 12 Dec 2005 19:19:49 GMT (74kb)

Title: Ultraviolet Radiation Constraints around the Circumstellar Habitable
Zones

Authors: Andrea P. Buccino, Guillermo A. Lemarchand, Pablo J. D. Mauas

Comments: 29 pages, 8 figures
\\
Ultraviolet radiation is known to inhibit photosynthesis, induce DNA
destruction and cause damage to a wide variety of proteins and lipids. In
particular, UV radiation between 200-300 nm becomes energetically very damaging to most of the terrestrial biological systems. On the other hand, UV radiation is usually considered one of the most important energy source on the primitive Earth for the synthesis of many biochemical compounds and, therefore, essential for several biogenesis processes. In this work, we use these properties of the UV radiation to define the bounderies of an ultraviolet habitable zone. We also analyze the evolution of the UV habitable zone during the main sequence stage of the star. We apply these criteria to study the UV habitable zone for those
extrasolar planetary systems that were observed by the International
Ultraviolet Explorer (IUE). We analyze the possibility that extrasolar planets
and moons could be suitable for life, according to the UV constrains presented
in this work and other accepted criteria of habitability (liquid water, orbital
stability, etc.).

\\ ( http://arXiv.org/abs/astro-ph/0512291 , 74kb)

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\\
Paper: astro-ph/0512292

Date: Mon, 12 Dec 2005 14:46:55 GMT (56kb)

Title: Gas Flow Across Gaps in Protoplanetary Disks

Authors: Steve H. Lubow and Gennaro D'Angelo

Comments: 10 pages, 3 figures, 1 table. To appear in The Astrophysical Journal
\\
We analyze the gas accretion flow through a planet-produced gap in a
protoplanetary disk. We adopt the alpha disk model and ignore effects of
planetary migration. We develop a semi-analytic, one-dimensional model that
accounts for the effects of the planet as a mass sink and also carry out
two-dimensional hydrodynamical simulations of a planet embedded in a disk. The
predictions of the mass flow rate through the gap based on the semi-analytic
model generally agree with the hydrodynamical simulations at the 25% level.
Through these models, we are able to explore steady state disk structures and
over large spatial ranges. The presence of an accreting Jupiter-mass planet
significantly lowers the density of the disk within a region of several times
the planet's orbital radius. The mass flow rate across the gap (and onto the
central star) is typically 10% to 25% of the mass accretion rate outside the
orbit of the planet, for planet-to-star mass ratios that range from 5e-5 to
1e-3.

\\ ( http://arXiv.org/abs/astro-ph/0512292 , 56kb)

[Guest_Richard Trigaux_*]

Paper: astro-ph/0512291

Ultraviolet radiation is known to inhibit photosynthesis, induce DNA
destruction and cause damage to a wide variety of proteins and lipids. In
particular, UV radiation between 200-300 nm becomes energetically very damaging to most of the terrestrial biological systems. On the other hand, UV radiation is usually considered one of the most important energy source on the primitive Earth for the synthesis of many biochemical compounds and, therefore, essential for several biogenesis processes. In this work, we use these properties of the UV radiation to define the bounderies of an ultraviolet habitable zone. We also analyze the evolution of the UV habitable zone during the main sequence stage of the star. We apply these criteria to study the UV habitable zone for those
extrasolar planetary systems that were observed by the International
Ultraviolet Explorer (IUE). We analyze the possibility that extrasolar planets
and moons could be suitable for life, according to the UV constrains presented
in this work and other accepted criteria of habitability (liquid water, orbital
stability, etc.).

\\ ( http://arXiv.org/abs/astro-ph/0512291 , 74kb)

------------------------------------------------------------------------------

Ultraviolet plays two opposite roles, fostering and destroying life. Large stars are ultra-violet rich, small red dwarves are UV poor. So there is still many work ahead to be able to predict the probability of life versus the size of the star. Perhaps this can be done only by observation of planet spectrums.

Anyway an environment must receive UVs, but the energetic/reactive molecules produced by UVs must be able to gather into shelterd places. This implies a complex environment, with at least two mediums.

[edstrick]

One thing I have not seen much discussion of, at least in scientifically literate public reporting, is the interplay between atmosphere column mass and habitabality. A planet at Mars distance from the sun with (arm waving number) 2 atmospheres surface pressure would have a stronger greenhouse than Earth and be more temperate than a 1 atmosphere pressure global Alaska world.

A relatively high mass planet with a dense atmosphere could orbit F type star and have a fairly well shielded surface, though it might end up in a runaway greenhouse too soon as the star brightened on the main sequence.

At the other end of star masses, a high mass terrestrial planet with thick atmosphere, if one can accrete near a red dwarf, would be protected by the thermal inertia of it's atmosphere from flare-star activity, and have much better dayside --> nightside heat transfer than a 1 atmosphere mass planet (since it's presumably tidally despun)

While humans might not tolerate a planet with say 5 atmospheres of nitrogen (nitrogen narcosis is not recommended), the "locals" that might evolve would have no problem.

[Guest_Richard Trigaux_*]

One thing I have not seen much discussion of, at least in scientifically literate public reporting, is the interplay between atmosphere column mass and habitabality.  A planet at Mars distance from the sun with (arm waving number) 2 atmospheres surface pressure would have a stronger greenhouse than Earth and be more temperate than a 1 atmosphere pressure global Alaska world. 

A relatively high mass planet with a dense atmosphere could orbit F type star and have a fairly well shielded surface, though it might end up in a runaway greenhouse too soon as the star brightened on the main sequence.

At the other end of star masses, a high mass terrestrial planet with thick atmosphere, if one can accrete near a red dwarf, would be protected by the thermal inertia of it's atmosphere from flare-star activity, and have much better dayside --> nightside heat transfer than a 1 atmosphere mass planet (since it's presumably tidally despun)

While humans might not tolerate a planet with say 5 atmospheres of nitrogen (nitrogen narcosis is not recommended), the "locals" that might evolve would have no problem.

Yes this is the question of the "ecosphere" of a star, the zone where life-prone climate is possible. This zone can be much larger than just one orbit in a Titus Bode repartition: far planets may have a stronger greenhouse effect. And we can assume that, most of the stars (if not all) have planets in orbits in a more or less Titus-Bode way, so that we alway find several planets into the ecosphere.


We can even imagine Titan with just more gas: its temperature would allow for Earth-like conditions. The problem is that planets so far from their star don't have the massive solar energy input which, on Earth, allows for a large biosphere, many ecological niches and thus a faster evolution. Titan with an Earth temperature would have developped only primitive bacteria.

Still better, planets like Earth have a self-regulation of greenhouse effect. It is a complex system involving biological reaction, and also geoghemical reactions. In gross there is a constant source of carbon dioxid (volcanoes) which increases greenhouse effect, and chemical reactions (deposit of limestones, chemical or biological) which efficiency is strongly increasing with temperature, so that greenhouse effect decreases with temperature. (but too slowly to counterbalance industrial gasses). So that there is a regulation at the actual temperature (such effects are the basis of the "Gaïa" metabolism). This process will allow still much more worlds to have life-prone conditions at a variety of distance of their star.

Nitrogen pressure is not much of a problem, as local people will be accustomed to it from their very appearance. Look like andean and tibetan people adapted to rare oxygen in only some thousands years.

[ljk4-1]

Paper: astro-ph/0512221
Date: Thu, 8 Dec 2005 15:11:22 GMT (163kb)

Title: Abundance ratios of volatile vs. refractory elements in
planet-harbouring stars: hints of pollution?

Authors: A. Ecuvillon (1), G. Israelian (1), N. C. Santos (2,3), M. Mayor (3),
G. Gilli (1,4) ((1) IAC, Spain, (2) Observatorio Astronomico de Lisboa,
Portugal, (3) Observatoire de Geneve, Switzerland, (4) Dipartimento di
Astronomia, Universita di Padova, Italy)

Comments: 10 pages, 7 figures, accepted for publication in A&A. Figures with
higher resolution are available at www.iac.es/proyect/abuntest
\\
We present the [X/H] trends as function of the elemental condensation
temperature Tc in 88 planet host stars and in a volume-limited comparison
sample of 33 dwarfs without detected planetary companions. We gathered
homogeneous abundance results for many volatile and refractory elements
spanning a wide range of Tc, from a few dozens to several hundreds kelvin. We
investigate possible anomalous trends of planet hosts with respect to
comparison sample stars in order to detect evidence of possible pollution
events. No significant differences are found in the behaviour of stars with and
without planets. This result is in agreement with a ``primordial'' origin of
the metal excess in planet host stars. However, a subgroup of 5 planet host and
1 comparison sample stars stands out for having particularly high [X/H] vs. Tc
slopes.

\\ ( http://arXiv.org/abs/astro-ph/0512221 , 163kb)

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\\
Paper: astro-ph/0512222
Date: Thu, 8 Dec 2005 15:16:52 GMT (42kb)

Title: Condensation temperature trends among stars with planets

Authors: Guillermo Gonzalez

Comments: 5 pages, 7 figures, MNRAS Letter in press
\\
Results from detailed spectroscopic analyses of stars hosting massive planets
are employed to search for trends between abundances and condensation
temperatures. The elements C, S, Na, Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Ni and
Zn are included in the analysis of 64 stars with planets and 33 comparison
stars. No significant trends are evident in the data. This null result suggests
that accretion of rocky material onto the photospheres of stars with planets is
not the primary explanation for their high metallicities. However, the
differences between the solar photospheric and meteoritic abundances do display
a weak but significant trend with condensation temperature. This suggests that
the metallicity of the sun's envelope has been enriched relative to its
interior by about 0.07 dex.

\\ ( http://arXiv.org/abs/astro-ph/0512222 , 42kb)

[ljk4-1]

Paper: astro-ph/0412356

replaced with revised version Tue, 13 Dec 2005 00:49:09 GMT (226kb)

Title: How Dry is the Brown Dwarf Desert?: Quantifying the Relative Number of
Planets, Brown Dwarfs and Stellar Companions around Nearby Sun-like Stars

Authors: Daniel Grether (School of Physics, University of New South Wales) and
Charles H. Lineweaver (Planetary Science Institute, Research School of
Astronomy and Astrophysics & Research School of Earth Sciences, Australian
National University)

Comments: Conforms to version accepted by ApJ. 13 pages formatted with
emulateapj.cls

\\ ( http://arXiv.org/abs/astro-ph/0412356 , 226kb)


Paper: astro-ph/0512322
Date: Tue, 13 Dec 2005 13:03:32 GMT (71kb)

Title: Spectroscopic companions of very young brown dwarfs

Authors: Joergens Viki (Leiden Observatory)

Comments: Proceeding of ESO workshop 'Multiple Stars across the HRD' (Garching
2005). 6 pages, 4 figures
\\
I review here the results of the first RV survey for spectroscopic companions
to very young brown dwarfs (BDs) and (very) low-mass stars in the ChaI
star-forming cloud with UVES at the VLT. This survey studies the binary
fraction in an as yet unexplored domain not only in terms of primary masses
(substellar regime) and ages (a few Myr) but also in terms of companion masses
(sensitive down to planetary masses) and separations (< 1 AU). The UVES spectra
obtained so far hint at spectroscopic companions of a few Jupiter masses around
one BD and around one low-mass star (M4.5) with orbital periods of at least
several months. Furthermore, the data indicate a multiplicity fraction
consistent with field BDs and stellar binaries for periods < 100 days.

\\ ( http://arXiv.org/abs/astro-ph/0512322 , 71kb)

[David]

Yes octopodes are bilaterals too, pardon my mistake. I was rather thinking to star fishes.

Surprising as it may seem, starfishes are bilaterians too! Not only that, but the echinoderms are more closely related to vertebrates like us than to other bilaterians like molluscs and insects. Starfish larvae are recognizably bilaterally symmetric, but as they mature they acquire the pentaradial symmetry characteristic of echinoderms; the radial symmetry is a secondary development, not shared by the common bilaterian ancestor.

[Guest_Richard Trigaux_*]

Surprising as it may seem, starfishes are bilaterians too!  Not only that, but the echinoderms are more closely related to vertebrates like us than to other bilaterians like molluscs and insects.  Starfish larvae are recognizably bilaterally symmetric, but as they mature they acquire the pentaradial symmetry characteristic of echinoderms; the radial symmetry is a secondary development, not shared by the common bilaterian ancestor.

Ahem ahem

there must be some beings who are not bilateralians, at last?

[ljk4-1]

Paper: astro-ph/0512371

Date: Wed, 14 Dec 2005 18:29:41 GMT (49kb)

Title: Polycyclic Aromatic Hydrocarbons Orbiting HD 233517, an Evolved
Oxygen-Rich Red Giant

Authors: M. Jura (UCLA), J. Bohac, B. Sargent, W. J. Forrest, J. Green, D. M.
Watson (Rochester), G. C. Sloan (Cornell), F. Markwick-Kemper (Virginia), C.
H. Chen, J. Najita (NOAO)

Comments: 11 pages, 2 figures, ApJ Letters, in press
\\
We report spectra obtained with the Spitzer Space Telescope in the 5 to 35
micron range of HD 233517, an evolved K2 III giant with circumstellar dust. At
wavelengths longer than 13 microns, the flux is a smooth continuum that varies
approximately as frequency to the -5/3 power. For wavelengths shorter than 13
microns, although the star is oxygen-rich, PAH features produced by carbon-rich
species at 6.3 microns, 8.2 microns, 11.3 microns and 12.7 microns are detected
along with likely broad silicate emission near 20 microns. These results can be
explained if there is a passive, flared disk orbiting HD 233517. Our data
support the hypothesis that organic molecules in orbiting disks may be
synthesized in situ as well as being incorporated from the interstellar medium.

\\ ( http://arXiv.org/abs/astro-ph/0512371 , 49kb)

[ljk4-1]

Computer Security Expert Rejects Hacker Hypothesis

For Immediate Release

LITTLE FERRY, NJ.., 15 December 2005 -- A member of the grassroots, nonprofit SETI League has allayed fears expressed by some scientists, that malevolent signals from extraterrestrial civilizations could cripple Earth's computer networks. Responding to the so-called "SETI Hacker Hypothesis" articulated by particle physicist Richard A. Carrigan, Jr. of the Fermi National Laboratory, Canadian computer security expert Marcus Leech states "we test some of the elements of this hypothesis and find them to be unsupportable."

SETI science seeks evidence of advanced technological civilizations in space, primarily by monitoring electromagnetic emissions with optical and radio telescopes. Through the SETI@home experiment at the University of California, Berkeley, millions of personal computers worldwide have since 1999 been participating in distributed processing of data from one of those telescopes, the 305 meter dish in Arecibo, Puerto Rico. The SETI Hacker Hypothesis suggests that those computers could be at risk from malicious code deliberately sent Earthward by extraterrestrials. But in a recent paper available on The SETI League website at http://www.setileague.org/articles/hacker.pdf, Leech concludes that these fears are unfounded.

Carrigan's concerns were first voiced in a paper presented at a 2002 BioAstronomy conference, titled "The Ultimate Hacker: SETI signals may need to be decontaminated." Elements of that paper have recently resurfaced on the self-proclaimed nerd website SlashDot.org, been reported in the British newspaper The Guardian and elsewhere, and submitted to the prestigious scholarly journal Acta Astronautica. Carrigan states that "Biological contamination from space is a remote but recognized possibility. SETI signals might also contain harmful information." He argues for a decontamination process similar to that used when the first moon rocks were returned to Earth in 1969.

In response, Leech, formerly security area director for the Internet Engineering Task Force, makes the most generous assumptions possible in favor of the alleged ET hacker, and shows that such beings would be unable to trigger a buffer overflow, the known security flaw in computers most often exploited by terrestrial hackers. He allows that "while one cannot recommend a cavalier attitude with respect to software quality used by our SETI researchers, it's a near-certainty that computer viruses from outer space will not be one of the threats that need to be defended against."

"We conclude," states Leech, "with apologies to the film Independence Day, that SETI-hacker scenarios are only plausible within the fanciful confines of Hollywood, and then only when our ET hackers happen to be Macintosh ™ savvy."

Largely using radio telescopes and optical telescopes, SETI scientists seek to determine whether humankind is alone in the universe. Since Congress terminated NASA's SETI funding in 1993, The SETI League and other scientific groups have privatized the research. Amateur and professional scientists interested in participating in the search for intelligent alien life, and citizens wishing to help support it, should email join @ setileague.org, check the SETI League Web site at http://www.setileague.org/, send a fax to +1 (201) 641-1771, or contact The SETI League, Inc. membership hotline at +1 (800) TAU-SETI. Be sure to provide us with a postal address to which we will mail further information. The SETI League, Inc. is a membership-supported, non-profit [501©(3)], educational and scientific corporation dedicated to the scientific Search for Extra-Terrestrial Intelligence.

P.S. Tearsheets are always appreciated. Thank you.

-end-

--
H. Paul Shuch, Ph.D. Executive Director, The SETI League, Inc.
433 Liberty Street, PO Box 555, Little Ferry NJ 07643 USA
voice (201) 641-1770; fax (201) 641-1771; URL http://www.setileague.org
email work: n6tx@setileague.org; home: drseti@cal.berkeley.edu

[David]

Ahem ahem

there must be some beings who are not bilateralians, at last? 

That's just the problem -- bilaterians have been so successful that they haven't left much room for other animals!

Non-bilaterian metazoans include:

motile jellyfish of various kinds (scyphozoan, hydrozoan and cubozoan medusae, and siphonophore and chondrophorine colony animals), all of which are cnidarians, and comb jellies which are in a different clade, the Ctenophora;

sessile corals, sea anemones, sea pens, and hydroids (which look misleadingly like plants), all cnidarians; and sponges, which are in their own clade, the Porifera.

Then there are a bunch of microscopic animals, some of which form macroscopic colonies that look like sticky green goo.

That pretty much sums it up for metazoans; the next closest related group of macroscopic organisms are the fungi. I have some doubts -- small ones -- about the ability of fungi-like organisms to create a space-faring civilization, but I am willing to be convinced otherwise.

[Guest_Richard Trigaux_*]

That pretty much sums it up for metazoans; the next closest related group of macroscopic organisms are the fungi.    I have some doubts -- small ones -- about the ability of fungi-like organisms to create a space-faring civilization, but I am willing to be convinced otherwise. 

fungi? Yes they can. At a similar state of evolution, we were not better. Given equal chances, they may have evolved in intelligent beings.

Seriously, what, I think, gave the advantage to bilateralians is not the bilateral symmetry by itself, it is a set of innovations the other were deprived, such as nervous cells, or the coelome (sorry this is the latin name I don't know in english) or closed inner body cavity which allowed all the building of further evolution. The lombrics can move thank to coelomes (one per ring), and we still have three body cavities (or lombric-like rings) separated by muscles: chest, belly, and a smaller perinear cavity. Fungi seriously lack such structures, and corals have an open boby cavity for feeding (the opening serving ans both mouth and anus).

These body structures were first coded by a series of adjacent genes, called homeobox, each one coding for a body segment, and controling the growth of each segment. Further evolution complexified this scheme, but without altering the basic structure. It is the very unlikely appearance of such a complex structure which took many time and was the real breakthrough which allowed bilateralians to be the first. As such an event is rare, it does not happens often, only once in Earth history, and it is likely not to happen again, as the bilateralians now hamper any further evolution of the other clades.

beings like fungi are multicellular, but they don't have a homeobox-like system, and other system such as specialized cells. This allow them to grow rapidly, but with only simple structures.

Sime dates for Earth history:
-multicellular beings are traced from -600 millions years and likely existed 1 billion years ago
-bilateralians are known since 600 millions years only.