Dinosaur extinction due to space dust?
June 14, 2009 12:40 PM   Subscribe

Could large land animals have died out due to the Earth's gravity increasing over millions of years?

Every year around 40,000 tons of mass are added to the earth by way of space dust. Perhaps when the massive dinosaurs walked the earth the earths gravity was in "golden window" that would allow such creatures to exist. Over the hundreds of millions of years they existed, could it be possible that the accumulation of dust and the increased gravity that mass brought with it have grown to the point where such giant beasts simply could no longer grow to the sizes they had in the past?

Or, is 40,000 tons of mass, even if it may have been higher in the early solar system, still insufficient to appreciably raise the mass of the earth to a noticeably higher gravity level?

This is something I've wondered about for years but never read anything that may have posed this question.
posted by OneCrayon to Science & Nature (10 answers total) 3 users marked this as a favorite
Earth's mass is about 7 x 10^21 tons. At 40000 tons per year, it would take about 10^17 years to double the earths mass. As the earth's only been around for about 4.5 billion years, space dust will have only increased its mass, and hence its gravity, by about 0.1 parts per million.
posted by pombe at 12:44 PM on June 14, 2009 [8 favorites]

Wolfram Alpha: No. Without checking your data, that would give a relative increase in gravity of about 0.000001 %.
posted by themel at 12:48 PM on June 14, 2009

Not to mention that any such significant increases in gravity over millions of years would have been compensated by evolutionary changes in bone strength and weight distribution.
posted by Rhaomi at 1:21 PM on June 14, 2009

Not to mention that any such significant increases in gravity over millions of years would have been compensated by evolutionary changes in bone strength and weight distribution.

Well, that was his point: that animals evolved over time to get smaller because gravity was increasing. Except, obviously, gravity wasn't increasing. Also, there was a time when there were Very large mammals as well. Not as large as dinosaurs., though.

I always though it had something to do with heat, like somehow larger animals would evolve when the planet was warmer.
posted by delmoi at 1:28 PM on June 14, 2009

There's an interesting thought here, but a few things:

1) The amount of time that organisms beyond simple one-celled or, if multi-celled, beyond bags of jelly, has only been ~600 million years. Any variation you're considering from space dust has to fit into this window, and not only that, it has to take into account several major extinction events from which many lineages disappeared or had to "start over"...which leads into...

2) An evolutionary progression called Cope's Law, which theorizes that lineages naturally evolve into bigger creatures with time - and in many cases you see the biggest creatures just before extinction. We see it with dinosaurs, we see it with mammals, we even see it with fossil plankton - again and again. For example, mammals have been getting bigger, and one of the biggest - whales - rival the dinosaurs in size. They've attained this in 65 million years, whereas the dinosaurs were around much longer than that and mammals have had to deal with considerably more climatic variation, too. As part of this, the time we are living in now is not the "end" of evolution by no means - it is a state, or a stage, in a much bigger story of which we cannot see the end. Perhaps mammals are on their way out, or they're on to much bigger and better things - we have no idea, just some educated guesses.

Also remember that in dinosaurs we also had species that were the size of chickens - how generalized or how specialized their ecological niches were also had a lot to do with size just as much as choice and availability of food (i.e. prey or predator). There's also latitudinal and elevation gradients, climate, and competition, etc. to consider. Some human morphologies are a great example of this: towards the equator we tend to get taller and skinner, and towards the poles we're short and more squat. Or consider the height variation you might see in a single generation with improved nutrition.

If you get into the big picture as you are, there is considerable astronomical variation which may affect sizes such as Milankovitch cycles (changes in the earth's orbit and tilt). For gravity, it would be interesting also to think about how close the earth is to the moon (the moon has been getting farther away with time). With space dust, you'd also have to consider how much dust is created or thrown out by large meteor impacts.

And, of course, proponents of either/both punctuated equilibrium (sudden evolutionary changes) or gradual change with time models would have plenty to say over your thought, too.
posted by barchan at 2:47 PM on June 14, 2009 [1 favorite]

Two things:
1) The range of size among dinosaurs was across the spectrum from very small to very large.

2) Significant numbers of plant and invertebrate families became extinct at about the same time, supporting the view that whatever caused the KT boundary was likely catastrophic.
posted by KirkJobSluder at 3:41 PM on June 14, 2009

The Earth's atmosphere has not always been 20% oxygen. Prior to the development of plants there was no free O2; during the time of the dinosaurs (250 to 65 Myr ago) there was up to 30% free oxygen in the atmosphere. I have heard this connected to giantism during those times, especially among insects.
posted by fantabulous timewaster at 9:48 PM on June 14, 2009

My link broke, sorry.
posted by themel at 1:32 AM on June 15, 2009

A good understanding of the physical factors that constrain animal size can be had from a marvelous little book (and companion PBS series) called The Search for Solutions, which can be bought used for $0.01.
posted by neuron at 6:02 AM on June 15, 2009

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