Seeing Forests in the Tree of Life

peter_wardPeter Ward, co-author of "Rare Earth," "The Life and Death of Planet Earth", and Professor of Geological Sciences at the University of Washington in Seattle

Peter Ward, a paleontologist and professor of geological sciences at the University of Washington in Seattle, has written extensively about the history of life on Earth, as well as the possibility for life elsewhere in the universe. His book "Rare Earth" has become a catch phrase among scientists, who fiercely debated the book’s hypothesis that simple, microbial life will be widespread in the universe, while complex animal or plant life will be rare.

Ward’s newest book, "Life As We Do Not Know It," will be published by Penguin’s Viking Press in November 2005. In a NASA Director’s Seminar on September 27, 2004, Ward outlined some of the ideas presented in his upcoming book.

This portion of the Director’s Seminar has been edited. It is published here with permission by Peter Ward.


"For the last few months I’ve been writing a book about life, called "Life As We Do Not Know It." In learning as much as I can about the chemistry of life, it has become clear to me that new taxonomic categories are warranted. The new tree of life I will propose here is not formalized, but at least I’ll throw out the idea.

tree_of_life
Traditional classification resembling a ‘tree of life’ organized into six kingdoms.
Credit: UCLA

Everyone since Darwin has said there’s only one kind of life on Earth — our good old DNA life. And yet, we do not know if that’s true on other planets.

So if there’s not DNA life, what else could there be? What are the alternative forms of chemistry? We might think about what chemistry permits, different information systems, different solvents, and different kinds of membranes.

Then we can think about the variable histories of life that a planet might have. The first is that life never evolves. The second is that it does evolve, it has a history, and then it dies out as the life systems age. That’s the one we think that Earth life has, as Don Brownlee and I showed in our book, "Life and Death of Planet Earth."

A third option is that life may get cut short by termination through mass extinction. Or life on a planet evolves, is exterminated, and then re-evolves. This might be the system that happened on Earth during the period of heavy bombardment, where we had impact after impact after impact; the impact frustration of life idea, where a life form comes about and then gets cleaned out. And finally, there’s Panspermia, where life is seeded from a planetary companion.

This takes us to a fascinating question of how many life forms can a planet have. Perhaps there are multiple forms or chemistries of life evolving, and then you have competition between them, and there’s one that wins and one that loses. Or multiple lines evolve, and more than one co-exists.

prokaryote
Prokaryotes are primitive cells, without a nucleus or membrane bound organelles, has DNA located in a "nuclear area", but the DNA is not bound inside the nucleus as in Eukaryotes. Prokaryotes have ribosomes, although the ribosomes are slightly more primitive than Eukaryotic cells.
Credit: OUC

The familiar tree of life is composed of three Domains: Archaea, Bacteria, and Eukaryotes. Where on this tree, and therefore where within current taxonomic practices, could you put something like RNA life? Would it be part of an existing Domain?

I’m not in any position to say when life on Earth moved from a RNA world to a DNA world. But the switch probably occurred because DNA is so much more efficient, due to copying fidelity. Steven Benner and Jack Szostak are working on artificially producing RNA segments, and their work has shown that RNA makes lots of mistakes in copying itself.

Their work is really what got me to start thinking about all this. Once these guys get artificial RNA life – and they will get it – where will we put it on our tree of life? If you put artificial animals like sheep and cows and dogs and humans on this tree, then we’re going to have to do the same for artificial RNA life. But I don’t think you can put it in a Domain of DNA life. When I start thinking about what RNA life may be, and what are the characteristics of DNA life, it suggests to me that our current tree is not inclusive enough.

Another case in point is viruses. Viruses are not alive. They do a lot of stuff to life, but they’re not alive. Every definition of life that you see excludes viruses because they cannot replicate themselves outside of the host, and therefore they fail the test of life.

Ok, but what about Giardia, or tapeworms, or any other parasite that can only replicate in a host? Those are alive, but viruses aren’t alive? Well, that’s what all biology texts tell you, because from Darwin on, there’s only one kind of life on this planet.

cell
In a eukaryote, the DNA is located in the nucleus of the cell. A DNA molecule is composed of two helically spiral strands, each composed of a linear chain of sugar and phosephate molecules.
Credit: MIT

So let’s do a little experiment that suggests that viruses are alive. Viruses as a group are thought to have a monophyletic origin: An ancestral virus arose, and everything evolved from that one virus. I would suggest that this is wrong. I’m going to stick my head way out on the chopping block, and suggest that perhaps some viruses are living fossils from the RNA world.

If we examine DNA and RNA viruses, we can see they have lots of different kinds of genomes. If we begin to look at their relative sizes and shapes, and the way they pack their genomes into them, the types of encapsulations they have, and the number of proteins they use, we see this whole world of stuff out there.

So I suggest we need to expand the taxonomic categories of life. We need a new higher level, which I call a Dominion. Let’s have a Dominion Terroa for Earth life, even though I’ve been told by my friend Roger Buick that it’s too close to "terrorist" and no one will like that.

But anyway, Earth life means life with DNA. DNA life is cellular or it’s encapsulated. It has a specific language, and codes for 20 amino acids. It’s a pretty specific type of life.

The second Dominion we could call the Dominion Ribosa, life with RNA only. I put this life within two groups: cellular and encapsulated. Cellular is totally extinct. Encapsulated are still with us: RNA viruses.

What kind of life can we expect to find on other planets? Perhaps DNA life will be common on Mars. We have tests that can find DNA life, but there may be RNA life that we would not find using current tests. As we head further out into the solar system, looking for life becomes even more difficult.

eukaryote
Eukaryotes probably emerged from prokaryotic ancestry about 1.6 – 2.1 billion years ago. The evolutionary diversification of eukaryotes has involved invention of organelles, and their modification.
Credit: UCLA

Although Jupiter’s moon Europa has liquid water, the cold temperature requires a non-water solvent for life. Reactions are so slow when you get down to minus 10 Centigrade that you are running out of room to make DNA life work. I followed up on work done by Chris Chyba, and I don’t think you can make Earth life work in the cold temperatures, not even in that surface slush ice where Chris would like to have life. I’ve tried to fit any Earth bacterium into that ice and make it work, and I can’t.

So once we get out to Jupiter, we’re going to need a solvent other than water, like ammonia. Here, you’re dealing with a life form that is very different from Earth life, and couldn’t live on Earth.

If we do find life on another planet and find out that it is not our kind of life, we’re going to need another taxonomic group. This will require us to think about forests of life, Arboreas.

We could have two very different life systems on Saturn’s moon Titan. On the surface there might be ethane seas, and then under that an ice layer, and beneath that we might have an ammonia ocean. An ammonia ocean would certainly be good for CHON life, or carbon-hydrogen-oxygen-nitrogen life, which is what I think most carbon-based life should be called. But on the surface, in these ethane seas, you can have silane life, life made from silicate compounds. So Titan could have two entirely different life chemistries co-existing.

We could also find silane life in the nitrogen geysers on Neptune’s moon Triton. Wouldn’t it be interesting if Triton and Titan, through Panspermia, exchanged life back and forth? That’s chemically permissible, and theoretically possible. If this is the case, then we’re going to see chemistry systems producing varieties of life through the planetary systems themselves.

This is why, at the end of this book I’ve just written, I’m suggesting that we think about manning a mission to Mars with a paleontologist, and a manning a mission to Titan with an organic chemist."


Related Web Pages

The Search for Life in the Universe
Lord of Gondwanaland
Rare Earth Debate Series
Interactive Presentation: The Life and Death of Planet Earth
Tree of Life
Eukaryotic Origins
The Tree of Life Web Project
First Complete Gorgon Fossil Found (ABC-AU)