t's advice worth remembering: Don't believe everything you read. No branch of learning is infallible, and even where the facts are more or less known and agreed, their reporting is subject to error, misinterpretation and the vagaries of politics and journalistic fashion. Incredible though it may seem, even "Lab Notes"—the science arm of USA Networks' eminently respectable SCIFI.com—occasionaly flubs.
Over the 42 columns we've run in this space since 1999, a handful of errors have crept in and been pointed out by readers. It is with rueful good humor (and an eye toward Scott's editorial last week on journalistic standards in the electronic press) that I post the following retractions. The mice with ears growing out of their backs ("Of Mighty Mice ... and Men?", April 2000) were the result of tissue engineering, not genetic engineering. Japan and Korea are not a part of the geographic feature known as the Sunda Shelf ("Kennewick Man", November 2000). Quantum dot solids are unlikely to be "far stronger than diamond" ("The Heart of the (Programmable) Matter", March 2001), although they could conceivably be fashioned from woven diamond fibers which may, under some circumstances, be slightly stronger than bulk diamond (just as fiberglass is stronger than glass). Spider-Man's Venom suit is not a nanotechnological artifact but a living alien creature ("The Physics of Spider-Man", May 2002). Also, the ganzfeld procedure for testing telepathy ("Psi Out", July 2002) was devised in 1974, not in the 1930s as stated.
Whew.
Two of the most significant errors I save for last, because their combined retraction is a fascinating story in its own right. First of all, in January 2000's "Global Warming, Hurrah!", I reported that the Earth had never experienced either a runaway greenhouse effect or its opposite: a runaway icehouse. This is incorrect, since over the course of the late 1990s, geologists were slowly concluding that an obscure theory, Brian Harland's 1964 "Snowball Earth" hypothesis, was probably correct. The debate still continues, but the majority opinion seems to be that, as in Robert Altman's 1979 dystopic film Quintet and Robert Silverberg's novel The Time of the Great Freeze, the Earth did actually freeze from pole to pole, not once but twice: first 2.5 billion years ago and again around 700 million years ago. The implications for life are both scary and wondrous, as we'll see in a minute.
Humanity survives to give itself a hand
The other major goof occurred earlier this year, in "Evolution and the Spaceport Bar", when I asserted that an evolving animal's new limbs usually begin as mouthparts and later migrate backward, and that unneeded limbs migrate even further back and "merge with the tail." Neither statement is correct; both result from a misunderstanding on my part. The second assertion is closer to the truth, since burrowing and swimming animals often lose their hind limbs in an effort to become more streamlined (the minimum-drag shape being a teardrop). But they don't ever combine with the tail.
In fact, the migration of limbs around the body occurs only in very limited ways, because multicellular animals with limbs are (with the exception of radially symmetric creatures like starfish) always segmented animals. That is, their bodies are composed of repeating units with varying patterns of growth. This segmentation is obvious in insects, whose worm-shaped larvae are divided into 18 distinct sections, which merge into the head (six segments), thorax (three segments) and abdomen (nine segments) of the adult. As an insect evolves over time, limbs (e.g., wings, antennae, mandibles) can appear and disappear on a given segment, or smoosh together as two segments merge into a single body structure, or move apart as segments grow. (In fact, there may be a tendency for multiple limb-bearing segments to crowd in toward the head and become mouthparts.) But segments can't simply disappear or change order, and a set of limbs cannot migrate from one segment to the next.
There is a good reason for this: Mutations which add extra copies of a gene are far less likely to be harmful than mutations which delete, alter or relocate it. Once the extra copy exists, it provides more opportunity for the creature to evolve in useful ways, and if the duplicated gene is a key developmental one (called a HOX gene), then the organism may develop an entirely new segment, which may provide a survival advantage.
So there's a tendency for a life form's complexity to increase over evolutionary time, until it reaches a saturation point where additional changes do more harm than good. At that point, small mutations of existing segments are much safer, likelier, and more successful than any radical change to the body plan. In fact, all the multicelled animals on Earth fall into 35 distinct body plans, and although there have been endless variations on these basic themes, no new plans have appeared on Earth in over 500 million years.
With our smooth skins it's not quite so obvious, but humans (who fall under the "tetrapod" or four-limbed vertebrate body plan) are also segmented. In fact, we have 38 HOX genes dividing us up, and a quick glance at our skeletons will show you that most of the segments they define consist of a vertebra or backbone, surrounded by tissue. Sixteen of these segments also include a pair of protective ribs to enclose the most delicate organs, and two of our segments sprout large limb pairs: our arms and legs. Several segments are fused to form our heads, and several more are fused at the other end to form the tailbone and pelvic girdle. The fundamental difference between insects and mammals is the number of segments, and how they grow.
So as you can see, segmentation is pretty darned important in the history of animal life here on Earth. It's what constrains us in our general shapes, while also allowing us to grow and shrink various anatomical details in response to a changing environment. This takes thousands or millions of years, but on the evolutionary/geological time scale, that's nothing. Really, we're remarkably editable.
But this segmentation gambit, which gave rise to most of the animal body plans we see today, almost didn't happen. We may have the Snowball Earth to thank for it.
The scenario goes something like this: For billions of years, life on Earth consisted of simple, single-celled bacteria, which fed on a variety of nutrients and energy sources. But some of them used sunlight to digest CO2, and produced a toxic and highly reactive waste product: oxygen. Most of this was absorbed by rocks, turning iron to rust, copper to copper oxide, and so on, giving rise to the various minerals we find in the planet's crust today. But around 2 billion years ago, a global calamity struck when the crust became saturated and could absorb no more oxygen. Instead, this poison began to build up in the atmosphere and oceans.
But this bacterial disaster opened a wider niche for an obscure line of single-celled organisms called eukaryotes, with complex features including a nucleus, a tough cell wall and, most importantly, the ability to tolerate oxygen, and even burn it as a high-energy fuel. Leaving almost no fossil traces behind them, these supercharged microorganisms came up with innovations like sexual reproduction, which allows for more rapid, flexible and successful evolution. They also invented a tough inner "cytoskeleton" which let them grow stronger and larger, and elaborate sets of chemical messengers which allowed nearby cells to communicate with each other, and thus to coordinate their development in useful ways.
Eventually, around 1 billion years ago, organized colonies of these creatures gave way to the first multicellular animals: probably microscopic sponges with an outer skin, a tissue layer, an inner digestive canal and (importantly) the capacity to evolve new tissue types, such as muscle. These early animals are also invisible to us, too small and soft to form good fossils, but during the Vendian period, from 700 million to 550 million years ago, some of them managed to develop the crucial gimmick of segmentation. This led—again, invisibly—to the first appearance of complex, adaptable body types, with the ready ability to grow and lose limbs, or to add new segments for greater complexity. But their development was sharply constrained by oxygen, which was still a trace gas in the oceans at that time.
Evolution courtesy of an uncommon cold
It might have gone on like that forever, with photosynthetic bacteria providing just enough oxygen for these primitive microbugs to survive on. If they produced any more, they would poison themselves—a powerful check on their own growth. The Earth was in balance.
But then something happened, perhaps a massive bloom and die-off which pulled greenhouse gases out of the atmosphere. Whatever it was, it sent a shock through the global climate, triggering the second Snowball Earth episode some 700 million years ago. The oceans froze over for perhaps as long as 30 million years, leaving plant life clinging to the underside of a global ice sheet, and animal life hunkering around volcanic vents in the sea floor, breathing what little oxygen drifted down from above. The evolutionary pressure on these isolated animal communities must have been enormous. Like many scientists, I'm frankly shocked that life survived at all, but at least 35 different animal species obviously did.
And when that deadly cap finally came off, as volcanic CO2 emissions heated the atmosphere again and melted away the ice, those plants and animals which survived had 50 to 70 million years of peace in which to quietly reconquer the oceans. But the balance had changed: photosynthetic bacteria were apparently edged out by photosynthetic eukaryotes—algae, the ancestors of modern plants—which were not as easily poisoned by their own wastes. Oxygen levels shot up dramatically in the air and seas, and the brakes on animal development abruptly vanished. Here the fossil record finally begins to show animals larger than one millimeter in size, with solid or semi-solid parts. The stage was set.
What happened next is so dramatic and sudden that paleontologists refer to it as the Cambrian Explosion. Beginning about 550 million years ago, all the tiny, marginal animals with their dozens of different body plans suddenly got big, and began filling every imaginable ecological niche. As the competition for resources heated up, some lines died out. Survival of the fittest. The others evolved and evolved and evolved some more, taking advantage of segmentation to sprout new limbs, new structures, new solutions to the problems of eating and mating and moving around.
All the creatures we see in the world today have recognizable ancestors in the Cambrian period. This is when complex animal life was invented and perfected. Since then, we've been rolling with the long, slow punches of climate change and the brief, sudden shock of mass extinctions. We've grown and shrunk, stretched and twisted, added a middle segment every now and then, but our fundamental body plans have changed very little. We are the children of the Cambrian.
So limbs do not and cannot migrate backward along our bodies, and the Earth is not immune to the occasional deep freeze. And indeed, it's a lucky thing I was wrong on both counts, or human beings could not have evolved at all, much less logged on to SCIFI.com to read these retractions. And that, I daresay, is an error we can all live with.
(With thanks to Peter Ward, Donald Bownlee, Wallace Arthur, PZ Myers, Damien Broderick, Geoffrey A. Landis, Foxspire@aol.com and ky@ma6.seikyou.ne.jp.)
Wil McCarthy is a rocket guidance engineer, robot designer, science fiction author and occasional aquanaut. He has contributed to three interplanetary spacecraft, five communication and weather satellites, a line of landmine-clearing robots, and some other "really cool stuff" he can't tell us about. His short fiction has graced the pages of Analog, Asimov's, Science Fiction Age and other major publications, and his novel-length works include Aggressor Six, the New York Times notable Bloom, and The Collapsium.
