Undeniable (11 page)

Darwin himself coined the phrase “tree of life,” after he had drawn a sketch depicting the branching points in the natural history of living things. As we study the relationships between species today, we see a great many more species in the present day than in the distant past. This is true even after taking into account the five (or six) major mass extinction events. What all this diversity and creation of new species implies is that you and I, as animals on the Tree of Life, are related to all kinds of other organisms that we might not expect to be related to.

For a century and a half, scientists of all stripes (biologists, paleontologists, archeologists, pathologists, immunologists, astrobiologists) have been classifying all of life, living and extinct, to fill in all the branches on that Tree of Life. You might think, then, that by this time we'd have it all sketched out. Certainly you'd expect that we'd all agree about the main branches. Well, we don't. But we're workin' on it.

With the discoveries in the 1970s of organisms that were heretofore unknown to science, we've had to rethink which living thing is related to which other living thing. For a long time, scientists generally agreed that there were animals and plants.
Wow, Bill. Thanks.
Seriously, and they are still often given the organizational designation: Kingdom Animalia and Kingdom Plantae, as we still Latinize biology words. But after thinking about this in terms of evolution—that is, in terms of which organism came into existence before which other organism—scientists realized that animals and plants have much more in common with each other than they (or we) have with almost all of the other living things here on Earth. It's a stunning fact. Most of the living things on our planet are microscopic, and those microscopic organisms are less like you than you are like a cabbage.

Scientists have climbed down the Tree of Life; or you might say that they've moved right to left on the time line of evolution. Either way, they've reevaluated who seems to be related to whom. As I write, we now consider nature to have given rise to three or four foundational types of living things, or domains of life. I'm going with four. We have Bacteria, Archaea (microbes that are fundamentally different from bacteria), Eukarya (that's us, animals and plants together), and Vira. You could also call that last one Viruses. (I took some Latin in school, and I prefer this style of pluralization for this particular second declension noun, describing this particular domain of living or nearly living things.)

Not everyone agrees with me that Vira deserve a domain. A traditional argument is that viruses are not really, fully living things. They need a host cell to reproduce; they cannot reproduce on their own. Viruses make no effort to take in their own nutrients. They do not maintain a steady metabolism. They remain intact for extraordinary periods with no interaction with their surroundings. No energy comes in or goes out.

My view is that viruses would not exist at all if we did not have other domains of living things for them to parasitize. Vira interact with the other forms of life, which makes them more like us than they are different, and certainly more like life than like nonlife. The recent discovery of giant viruses like Mimivirus and Pandoravirus, so large that they blur the line between virus and bacterium, strengthens the case. At the same time, Vira clearly do not belong to any of the other three domains. For me, they should have their own significant branch on the Tree of Life.

After we've sorted out life's domains, it becomes a bit tangled as to whether we should continue to go with the old, increasingly fine designations. If we did, it would go domain, kingdom, phylum, class, order, family, genus, and species. O, would but it were that tidy. By diligently investigating the DNA and cellular structure of organisms, scientists have come up with terms like superfamily, subfamily, and infrakingdom. These words have been coined just to help investigators make sense of it all. I'll spare us the tangled details.

For me, the most important, and most confusing, part of this story is the division of Archaea and Bacteria into separate domains of life. It is a division that biologists recognized only within the past three decades. You are, no doubt, familiar with bacteria. Archaea are microscopic, like bacteria, and even resemble bacteria when magnified. But inside, they are significantly different. Of the many, many microscopic organisms, some have nuclei in their cells, and some don't. Of the ones without nuclei, some use several proteins in sequence to process or metabolize chemicals around them for energy. Some use about half as many proteins in a simpler sequence to do the same job. The simpler ones are now considered to belong to Domain Bacteria. The more complicated ones belong to Domain Archaea. Also, the Archaea have a more complicated or in some cases just thicker membrane of proteins and lipids around them than do the Bacteria. The Vira remain their own deal.

For people who study these domains, the difference is clear. Both the Bacteria and the Archaea do not have nuclei. Whereas, you and I do … in our cells. That's why we are said to be Eukaryotes; it's from Greek words meaning “having nut” (having nucleus). The Archaea and Bacteria are Prokaryotes, meaning “before nut” (before nucleus). This turns out to be of astonishing significance in the natural history of living things.

The next steps in classification become a bit blurry as scientists research and ponder the nature of our relationship to everybody else—that is to every other living thing here on Earth. If we continue to dabble in the term
kingdom
, one level of hierarchy below the domain, we get five or so. Everyone agrees that there are fungi. They get their own kingdom, the Fungi. Then there is another group called the protists that are often classified in Kingdom Protista (the malaria parasite belongs to this group). After that we have the single-celled Kingdom of Eubacteria. The eu- prefix designates the true bacteria. Then come prokaryotes—no nuclei inside. Then plants, the Plantae. And, the animals, like you and me, the Animalia.

In this way of looking at things, the least understood is the Kingdom of the Archaea. And yet they may be where life all started. It is an amazing insight.

If this version of the Tree of Life seems a little complicated, it is. But I must add: It's also just cool. Living things live and carry on with their business of living and reproducing whether or not you and I have a clue as to who gave rise to whom, or where the Vira fit in. But with the new genetic tools that let biologists assay the sequence of amino acids on DNA and RNA molecules, we are closer than ever to understanding which type of living thing came first. Then we can meaningfully ask a whole new set of questions.

Here's a big one: Why did all life apparently descend from just a single ancestor? Did some other type of primordial life take a shot at living and reproducing, but it just couldn't keep up? It's possible that life started more than once, and that you and I are the result of an ancient sorting out. I'll circle back to this idea near the end of the book in chapter 36.

 

12

BIODIVERSITY COMES IN THE TERRITORY

In recent years, scientists have devoted a great deal of effort to studying Earth's biodiversity, the total variety of life. Most often, people talk about biodiversity in terms of ecology and conservation, but there is much more to it. Biodiversity can be quantified. It is a measure of the results of evolution. It is like a master index of all the populations of all the species that have come into existence today and all those that have been lost to extinction.

When we look at the fossil record, we see that biodiversity has been generally on the rise since the beginning of life some 3.5 billion years ago: The Tree of Life keeps getting bushier. If indeed we all are descendants of a common ancestor, this is just what we would expect. With every reproduction, there is a chance for a mutation that may or may not prove to be beneficial to the offspring. If it's beneficial, that organism, along with its genes, survives well enough to reproduce, passing its genes forward one more generation. With this happening over and over and over, all day, all the time, around Earth, we end up with species after species distributed everywhere.

Confirming that trend was not a foregone conclusion. If the world and all its species of animals and plants were created at once by some supernatural force or event, we might expect nothing but the fossils of familiar, living species as we dig down in Earth's crust. Or if there was a before-and-after transition, as is described in
The Bible
, we might expect a great many fossils of now-extinct species in lower layers or strata of rocky formations, then a sudden break (corresponding to the end of Eden, or perhaps Noah's flood), followed by only modern species in more recent strata. But that is not what we observe, not even remotely.

There is no such division between pre-flood fossils and post-flood fossils, or pre-Eden and post-Eden ones. What we see instead is a continuum, one that shows the gradual trend toward increased biodiversity. That's exactly what Darwinian evolution predicts. Replication after replication, with chance after chance of small changes, leads to a gradual spread of good-enough traits that help a species compete. The opening of relatively unpopulated environments and the isolation of small populations encourage the emergence of new species. With more different kinds of living things coming into existence, more and different environments and energy resources could be exploited. Near as we can tell, what slows the natural increase in biodiversity is catastrophes like asteroids hitting Earth, smaller extinction events, like countywide mudslides, and, of course, us. Humans are apparently causing a mass extinction right now.

The evidence isn't flawless, of course. When we look at the fossils we have found all over the world and compile the fossil record, we have to expect limitations. After all, if a group of organisms was buried and eventually fossilized three billion years ago, we would expect fewer fossils to survive the trip through time. Tectonic plates shift and slide. Landscapes dry out and get flooded, often repeatedly. Any fossilized remains have a much greater chance of getting damaged, destroyed, or dissolved the longer they are buried in the earth. Nevertheless even with that taken into account, we find that there is more diversity among the more recent fossils.

The driver of all this diversity is energy. In science education we like to say that energy is what makes things go, run, or happen. So it is with living systems. They (we) need energy to live, to move, to grow, and reproduce. If we ask ourselves, “Where is there the most energy available to living things?” we come up with two sources at least. The first is sunlight. The second is the primordial energy of Earth's interior. The same is probably true of life on other worlds (if it exists), as I'll discuss later.

Whether you're a green plant or something that eats green plants, or something that eats things that eat green plants, you are going to find the most sunlight near the equator. No surprise then that there are more living things in a tropical rain forest and around tropical coral reefs than there are on the open ice of the Arctic and Antarctic, human scientists stationed there notwithstanding. But along with the multitude of individual living things, there are also more different kinds of living things near the equator than we observe in the north and south polar regions.

Furthermore, there is a gradient of diversity. There are more different kinds of living things per square meter or hectare or square yard in the Amazonian rain forest than in the rain forest of Belize or Guatemala. They each, in turn, have more diversity than we find in the boreal forests of Northern Canada. There is a little more diversity on New Zealand's North Island, which is closer to the equator than New Zealand's South Island, which is closer to the South Pole. There are other local factors, rainfall especially, but the trend holds in the big picture. If you think about it, you may have observed evidence of this phenomenon yourself. If you live in or visit the U.S., compare the thick growth in the wetlands of Louisiana with the headwaters of the Mississippi River in Minnesota.

The areas of Earth with the most energy input are also the areas with the most biodiversity. This gradient from more to less as you go north or south from the equator is yet further evidence of evolution. These ecosystems have been there for a long, long time, and the longer an ecosystem is running, the more living things can multiply. As they increase in number, they'll carry more mutations and more variants. With diversity in offspring and lots and lots of time, we end up with a biodiverse ecosystem.

The patterns of energy are very different for living things that rely on Earth's internal energy. Nuclear fission of natural radioactive elements like uranium and thorium keeps the insides of the planet molten. We experience that energy directly when that heat finds its way up close to the surface. It drives steam geysers, deep-ocean vents, volcanoes, and earthquakes. But over the past few decades, scientists have discovered that it also drives whole ecosystems at the bottom of the ocean, a previously unrecognized realm of biodiversity.