Comment by api | Hacker News Reader

Comment by api | Hacker News Reader

I'm a fan of the view that evolution should be viewed as an information transfer process. It transfers information about the fitness landscape into the genome.

Similar environments will give rise then to similar information. There will be variance, of course, but it's going to be a lot narrower than the random walk interpretation of evolution would predict.

There is evidence for this from several places:

* Convergent evolution on Earth at many scales

* Digital evolution experiments showing similar endpoints in similar environments and against similar fitness landscapes

* ... and now this series of experiments showing convergent evolution in the lab even down to the genetic level

There's a lot of interesting implications here. For example: can we really say that another planet very similar to Earth that is of similar age might not harbor another bipedal warm blooded sentient species physically similar to ourselves? I'd be really surprised if we found Star Trek levels of similarity (e.g. biological and sexual compatibility), but some level of similarity in areas like basic anatomical structure would not shock me. Perhaps the anthropomorphic alien depictions in sci-fi (Giger's alien, the "greys," etc.) are not terribly implausible.

Of course a radically different world harboring life might yield something radically different for similar reasons.

I think a lot of it boils down to what the evolutionary pressures (defn: [1]) on another planet might look like. If we discover life on another planet, it wouldn't surprise me if some of the organisms we find resemble organisms in some branches of the "tree of life" on Earth, but I think it would be unlikely that there would be so many similar evolutionary pressures that we would see something particularly similar to ourselves.

For example, life has been around for at least 3.5 billion years on this planet[2]. At this point in time, there is no evidence that any other species on this planet has ever had a technological civilization that would be of particular interest to us in the way that is often envisioned for hypothetical alien visitors. This isn't for lack of different species[3] or for insufficient genomic complexity[4] in other species. In other words, there's nothing about the human genome that represents a "pinnacle" of evolution, as much as we tend to hold ourselves in pretty high regard among other species.

As another example, let's walk the tree of life (http://tolweb.org/Life_on_Earth/1) to get a ballpark idea of how many different features we run into before encountering homo sapiens:

1. Eukaryotes (organisms with nucleated cells) http://tolweb.org/Eukaryotes/3

2. Animals (multicellular, heterotrophic, lack rigid cell walls, motile, most have embryonic blastula stage) http://tolweb.org/Animals/2374

3. Bilateria (bilateral symmetry with three germ layers) http://tolweb.org/Bilateria/2459

4. Deuterostomes (blastopore becomes the anus during development) http://tolweb.org/Deuterostomia/2466

5. Chordates (animals possessing a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail for at least some period of their life cycles) http://tolweb.org/Chordata/2499

6. Craniata (have hard bone or cartilage skull) http://tolweb.org/Craniata/14826

7. Vertebrata (have backbones) http://tolweb.org/Vertebrata/14829

8. Gnathostomata (have jaws) http://tolweb.org/Gnathostomata/14843

9. Sarcopterygii (lobe-finned fish & terrestrial vertebrates. I am not an evolutionary biologist but it seems like this roughly corresponds to the evolution of limb-like structures) http://tolweb.org/Sarcopterygii/14922

10. Tetrapods (more properly terrestrial vertebrates, but to simplify things a little. Basically 4 limbs) http://tolweb.org/Terrestrial_Vertebrates/14952

11. Amniota (have eggs equipped with an amnios) http://tolweb.org/Amniota/14990

12. Synapsids (have a temporal fenestra) http://tolweb.org/Synapsida/14845

13. Therapsids (have particular skeletal and muscular structures) http://tolweb.org/Therapsida/14973

14. Mammals (hair, three middle ear bones, mammary glands, and a neocortex) http://tolweb.org/Mammalia/15040

15. Eutheria (have particular skeletal structures--not strictly placental) http://tolweb.org/Eutheria/15997

16. Primates http://tolweb.org/Primates/15963

There are several more branches before getting to homo sapiens.

I think the big remaining questions are:

1. What features of an organism are necessary for that species to develop a technological spacefaring civilization?

2. Are evolutionary pressures that support the development of those features likely to appear eventually on life-bearing planets similar to Earth? (I think this assumes that abiogenesis on other planets would be likely to produce simple cellular structures with similar capacities for evolution.)

3. Are we in a simulation created by another species in a "real" universe to answer similar questions? :)

I'd guess...

1. Limbs (and the supporting underlying physical structures) with significant capacity for manipulating objects, terrestrial-dwelling (as opposed to living in the sea), a advanced neocortex, and way too much free time, along with very difficult-to-impress mating prospects.

2. Depends on how similar to earth (a waterworld would be unlikely to have any species developing advanced technology, I think... maybe I'm not being imaginative enough). Also depends on how many of the evolutionary pressures that got us to this point were more or less inevitable as opposed to random. It seems that even if it's relatively likely that a species similar to humans will evolve on planets similar to Earth, there are still numerous additional hurdles in the development of different kinds of societies that could make or break the whole spacefaring thing. Even now, humans are not a spacefaring species, and the odds that we nuke ourselves out of existence or create a genetically engineered superbug that wipes us all out are relatively high considering we have absolutely no backup plan as a species for either of those events (or any number of other catastrophic global events). So maybe my answer to #1 is insufficient because we're not really a spacefaring species yet.

3. If they have that kind of computing power, I would've expected something more interesting. Maybe they're trying to predict the odds of encountering other species like themselves during a galactic collision? :) https://en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_co...

[1]: https://en.wikipedia.org/wiki/Evolutionary_pressure

[2]: https://en.wikipedia.org/wiki/Abiogenesis

[3]: There are approximately 10 million species currently alive on Earth (more or less a minimum bound, the actual number may be much higher). http://www.nytimes.com/2011/08/30/science/30species.html?_r=... At least 99% of all species to have ever lived are extinct now: http://www.physicalgeography.net/fundamentals/9h.html

[4]: http://en.wikipedia.org/wiki/Genome#Genome_size

I have to differ.

All of life on earth operates using mostly the same biochemistry. While there are countless differences and exceptions (eg. different codons, different genome architecture, metabolic and gene product differences), they are vastly outweighed by the vast magnitude of commonality. This is mostly observable at the small scale (nucleic acid, protein, lipid chemistries) but also somewhat true at a larger scale (certain transport processes, etc).

Bacteria have an incredible amount of biochemistry in common with us. And so do all other living things. Bacteria will of course be radically different when we contrast ourselves to other eukaryotes, but we can still see our biochemistry in them.

We all evolved from the same lineage, so all of the "core architectural" decisions were made long ago--before prokaryotes even came into being. The earliest choices made in exploring the chemical-physical fitness landscape turned out to have the greatest impact; all of our biology is expressed in terms of the primitives that were first established. For example, the universal adoption of the amino acid L-isomers. It's a seemingly arbitrary choice of little importance, but once that choice was made, earth biology forever had to live with it.

If there were different paths taken early on, metabolism would be different, polymerization would be different--cellular replication itself might also be different (one can only imagine). I imagine there would be cells. At higher levels, the choice of "building blocks" has an impact on bioinorganic chemistries (think heme groups). How you chelate depends on structure.

I assume that the heme group chemistry only evolved because of a highly oxygenated environment that life could exploit for energy production. Had our aqueous environment (and later atmosphere) been of a different composition, oxygen may not have played the same role.

Further tangent: certain elements are extremely important. Organic (carbon) chemistry is incredible due to the valence. Evolving on a world without oxygen might really suck, because oxygen chemistry is just so perfect for redox. And what of all the other important chemistries? Do they have a bioavailable source?

Anyhow, it's suffice to say that any change in early evolutionary choices would have far-reaching impact on the essential reactions (energetics, thermo, kinetics, dynamics...) and might yield results that are altogether alien to us from an Earth biology standpoint. Once you have your building blocks, you can't exactly backtrack.

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