Darwin blogging: Evolution in Astrophysics

In honor of Charles Darwin's 200th birthday a few days ago (February 12th) I thought it might be fun to consider the concept of evolution as it appears in astrophysics. Several rather fundamental concepts in modern astrophysics have the word evolution in them, and we're not even talking about astrobiology either.

What are these concepts and why (and how) are we using the word evolution to describe them?

Lets start by making it clear that evolution, when used in astrophysics, is largely used in the sense "change with time" and perhaps, at a stretch, "descent with modification." We don't mean "change in gene frequencies within a population of organisms" or natural selection.

There are three major core concepts in modern astrophysics that bear the name evolution:

1. Stellar Evolution: The set of physical changes and stages a star (usually a single star) undergoes throughout its lifetime. This includes its formation, ignition of nuclear hydrogen burning at the start of its Main Sequence life, the Main Sequence, and post Main Sequence life as core hydrogen burning ceases. For some classes of stars there will be mass loss via stellar winds or expulsive processes, and some stars end up as white dwarfs, neutron stars, or even black holes. The details depend on the initial mass of the star, its initial elemental composition (often termed chemical composition), whether its part of a binary stellar system (even more complicated than single star stellar evolution), and in some cases its environment (stellar evolution in dense star clusters has additional complications, including stellar collisions).
2. Chemical Evolution: The changes in the chemical composition of the Universe (in particular stars and galaxies, but also interstellar and intergalactic gas) as a result of nucleosynthesis within stars, and the expulsion of newly created elements in stellar winds, novae, and supernovae. Supernovae do more than just expel elements but also explosively synthesize elements that normal stellar nuclear fusion can not produce. The expelled elements usually end up in the interstellar medium (ISM), from which subsequent generations of stars form. The details of stellar evolution depend on the initial composition of the stars, so chemical evolution affects stellar evolution, which in turn drives chemical evolution, and so on...
3. Galaxy Evolution: The processes by which galaxies form and change with time, which are influenced by their environment within the inter-galactic medium (IGM) and neighboring galaxies, their gas content (fuel to create new stars with), the history of star formation within them (in turn affecting their own chemical evolution), and the presence and role of and Active Galactic Nuclei. Galaxies certainly grow and change with time, and to a limit extent events can change some of them one from one type galaxy to another (e.g. mergers of spiral galaxies can eventually create elliptical galaxies).

This is a very superficial outline of stellar, chemical and galaxy evolution, and when played out 13.5 billion years you can imagine the interconnections between all these processes can generate a fair bit of complexity. Nevertheless, this complexity falls far short of the complexity and interconnectedness of biology, and none of these processes approach the biological concept of evolution. So why call them evolution?

One could try to argue that astrophysicists are simply using the older meaning of evolution as change or progression. But that explanation seems contrived, as all of these astrophysical concepts were developed well after Darwin's "Origin of Species" came out in 1859. Indeed, the leading British (*) physicist Lord Kelvin (William Thomson) caused Darwin and his supporters much anguish as Kevin's (incorrect) theory for the age of the sun allowed only 20 million years for the age of the Solar System, seemingly too little time for evolution to have happened.

More recognizably modern theories of stellar structure, nucleosynthesis and stellar evolution only followed the development of quantum mechanics in the first decades of the 20th century. The big names in the development of these astrophysical theories are Eddington, Jeans, Milne, Chandrasekhar (**), Bethe, Gamow, Fowler and the (rather tragic) Fred Hoyle. In short, the foundation of what is now termed stellar evolutionary theory and chemical evolution was developed between ~1920 an ~1960.

The concept of Galaxies as separate entities also date from about this time. The famous Shapley - Curtis debate happened in 1920, so the issue had been brewing for a little while before that (Virginia Trimble has a nice discussion of the history, background and outcome of this debate). Hubble's work, demonstrating the reality of galaxies other than our own, and the expanding Universe, also date to the 1920's.

I would suspect that the reason astrophysicists use the word evolution to describe many of the core concepts of modern astrophysics has a lot to do with the great influence and power biological evolution has a scientific concept. In short, the biological theory of evolution is such a elegant, beautiful and powerful bit of science that we astrophysicists are happy, even eager, to associate the word with the theories we consider to be some of the most fundamental, powerful, elegant and important aspects of our own science.

This is of course pure speculation on my part. But I would like to think that naming these theories stellar evolution, chemical evolution and galaxy evolution is (perhaps unconsciously) the tribute we astrophysicists pay to the great naturalist and scientist Charles Darwin.

(*) Kelvin was an Ulster Scot. Ireland was at that time still part of the U.K.
(**) The Chandra X-ray Observatory (which I use a lot) and the Chandra Postdoctorall Fellowship that I had are all named after Chandrasekhar.

Feathers and Flares

Haven't had time to get back to the second Hoyle post, but on the subject I saw "The Dinosaur Feather Mystery" on the Science Channel and was blown away by the progress that has been made in both paleontology and evo-devo in the last few years on the evolution of feathers and flight. And there I was still thinking that feathers were modified scales...

Other interesting stuff to watch: a movie of the variable X-ray sky covering 1996-1999, based on RXTE, as linked to by AstroDyke.

Asteroid impacts might be beneficial (in the long term)

The image on the left is an artists impression (from http://universe-review.ca/R10-19-animals.htm) of the first moments of the K-T impact, the asteroid impact that probably caused the mass extinction event at the end of the Cretaceous period about 65 million years ago.

Although mass extinctions wipe out many of the species that are present on Earth at the time of the extinction event, the number and diversity of animal and plant species ultimately increases after mass extinctions. Thus, although mass extinctions are in the short term (several million years) destructive, on the longer terms of hundreds of millions of years they may actually beneficial. Carl Zimmer discusses this, more specifically asteroid-related extinctions, in an article at Wired (NB, only a few of the documented mass extinctions have been plausibly shown to be due to asteroid impact).

The ambiguous and poorly-understood long-term effect of destructive astronomical events (such as asteroid impacts, supernovae, gamma-ray bursts, AGN, etc) on biological life is something that renders assessing the size of the galactic habitable zone difficult, if not impossible, at the present and with our current understanding of these astronomical events and the fossil record.

In order to arrive at a conception of a very small Galactic Habitable Zone (visible in the images from the 2001 Gonzalez et al Scientific American article) , i.e. that the Solar system and the Earth was very unusual (a "privileged planet" that was then evidence of divine favor [or in code: Intelligent Design]), Guillermo Gonzalez assumed that anything that increases the chances of asteroid impacts, or nearby supernova, or high UV or cosmic ray fluxes, was negative and harmful to life.

But those assumptions are by no means robustly justified by existing data, nor are they unique (as the opposite effect, as discussed in Zimmer's article, could plausibly be true), as other astrobiologists pointed out the him at the time. Indeed, it could be that external events are ultimately responsible for driving greater evolutionary diversity and hence for increasing the chances of complex multi-cellular life evolving.

While on the subject of asteroid impacts, SPACE.com has two asteroid-related articles.

The first article, by Charles Choi, discusses new simulations by researchers at Sandia National Labs that suggest that the 1908 Tunguska explosion could have been caused by meteorite only 20 meters in diameter, smaller that previously thought. As there the number of asteroids of a given size is a strongly decreasing function of the their size., this implies that Tunguska-level events might be more common than previously thought. However, the actual destruction caused by the Tunguska explosion is also probably less than previously estimated, so rest easy!


The second article, by Alicia Chang, discusses an asteroid that has a 1 in 75 chance of hitting Mars (not us) this coming January 30th. This asteroid, 2007 WD5, would also cause a Tunguska-level explosion, equivalent to an explosion of about 15 Megatons of TNT.

The harvestmen of deep time

[image source: Gonzalo Giribet / NY times]

Carl Zimmer has produced yet another fascinating article in the science section of the NYT, featuring Dr Gonzalo Giribet and his group's research on the evolution of mite harvestmen (a relative of the daddy longlegs).

In short, as any species of mite harvestmen has a small physical range (apparently of order 50 miles or so) and hence do not spread or disperse much on their own, they provide a great way of tracing continental motion on time scales of hundreds of millions of years.

New Homo Erectus and Homo Habilis fossil finds

To get beyond the somewhat distorted press accounts regarding the Spoor et al letter in Nature on "Implications of new early Homo fossils from Ileret, east of Lake Turkana, Kenya" you should read this post at John Hawk's anthropology web log.

As with much press coverage of scientific issues something that has been known about for many decades: that multiple species of Homo coexisted at the same time, and that the evolutionary tree of genus Homo is quite bushy and complicated, and not a simple linear ladder of "progress"; is being presented as a new discovery, and furthermore the actual issues discussed in the Spoor letter aren't covered in the press coverage.

The image is a to-scale superposition of the skulls of the young adult (or late subadult) Homo Erectus (KNM-ER 42700, one of the subjects of the Spoor et al letter, cranial volume about 700 ml) on top of the skull of the largest known Homo Erectus skull (OH 9). A cool image that illustrates the diversity within Homo Erectus.

Just FYI, 90% of modern humans have cranial volumes in the range 1040 to 1595 ml [talk.origins FAQ], with volumes less than 1000 ml being extremely uncommon.

Coelacanths and evolution

PZ Myers has an interesting post on Coelacanths and the one remaining member of their once-great order, Latimeria, discussing the amusingly/annoyingly stupid arguments Creationists use when trying to claim that Coelacanths disprove evolution.


I try not to post on evolution and biology too much, given that there are so many blogs on the subject by professionals, but in the end I decided this was interesting and peculiar enough to warrant a link.

Did a starburst in the Milky Way 2.4 billion years ago have affect life on Earth?

2.4 billion years ago a storm of Cosmic Rays produced by an increase in the star formation rate of our home galaxy (the Milky Way) ionized the Earth's atmosphere to such an extent it changed the climate, triggering spurts of growth and die-backs in the primitive life on the Earth at that time.

That is the sure-to-be-highly-controversial suggestion in a new paper by Danish scientists (H. Svensmark, 2006, Astronomische Nachrichten, 27, 871) reported on by space.com. When I get back from the honeymoon I'll have to check out the full paper and write a more detailed follow up of this.

As a starburst guy I've love for this to be true, but right now it is best to be skeptical. That Cosmic Rays (CR) strongly affect the Earth climate appears to be by no means well established, and has often been used by climate-change deniers (e.g. see this older post of mine about CR-driven climate change claims made by [strangely enough!] the exact same Danish group).

Also there are issues about the significance, duration, and effect any upturn in Galactic star formation at that time may have been - even if there were a major burst in the MW its by no means clear how the CR flux at Earth would have been altered.

I'll look into these and other issues later...

[update 14/12/06 (1) I still haven't got around to reading the paper in full. (2) Typo in title has been corrected]

Fascinating Fungi

The ever interesting Carl Zimmer has a fascinating post up about Fungi. Go read.

[Update: picture from a hike we did in Pennsylvania back in 2003.]