The remora is so ridiculous that no one would try to make it up. The top of its head is a giant, flat suction cup. It uses the cup to lock onto the bodies of bigger animals, such as sharks, sea turtles, and whales. As the big animal swims for miles in search of a meal, the remora hangs on for the ride. When its host finds a victim, the remora detaches and feasts on the remains. It sometimes cleans its host’s body and mouth of parasites, and then clamps its head back on for another ride.

The remora’s ridiculousness makes it a fascinating evolutionary puzzle just waiting for the solving. Other species clamp themselves onto other animals–whale barnacles, for example, grow prongs from their shells that anchor them to whale skin–but among fish, remoras are exceptional. Their closest relatives include Mahi-Mahi and amberjacks, neither of which has anything on their head that even faintly resembles the remora’s sucker. Only after the ancestors of remoras and these ordinary fish split apart some 50 million years ago, the remoras evolved a remarkably new piece of anatomy.

When you look closely at the remora’s suction disk, its remarkableness only grows. It looks like a spiked Venetian blind. Pairs of slat-like bones called lamellae form a series of rows running down the length of its head, and muscles running from the remora’s skull to those bones pivot them, creating spaces between the rows.

That negative pressure pulls the remora towards its host’s body. Each lamella also has a comb-like set of pins that help make its clamp even more secure. The whole structure is surrounding by a loose fleshy lip, ensuring that no water slips in, keeping the seal tight.

As a result, remoras can create a vacuum that’s not just strong enough to attach them to an animal, but to stay attached as water rushes past them. They can even hold tight as their hosts try to scrape them off on rocks. But a remora can instantly release itself when it’s time to eat, with just a flick of its muscles.

As impressive as the remora’s suction disk may be, however, it’s not actually all that new. As is so often the case in nature, it’s actually just evolutionary tinkering with old parts.

Scientists have gotten some clues to the remora’s origins by looking at how they grow up. When remoras hatch, they don’t yet have a suction disk. Last year Ralf Britz of the Natural History Museum in London and David Johnson of the Smithsonian made a careful study of young remoras to document their development.  They found that the bones and muscles of the remora’s sucker start out much like the bones and muscles in a fin found on the back of other fish, known as the dorsal fin. They develop in the same location and have the same structure. But later, the bones and muscles move forward to the head.

They also change shape along the way. The fin spines spread out into lamellae that sprout a comb of spikes. In ordinary fish, the fin spine sits atop a small round bone. In remoras, that small bone widens out into another set of lamellae.

While the development of embryos doesn’t recapitulate evolution, it can offer some hints about how new things evolved from old ones. Britz and Johnson’s research indicates that the remora suction disk started out, improbably enough, as a dorsal fin. The fin stretched out into a complex vacuum device and moved up to the head. The underlying similarity between sucking disks and fins only becomes clear when you see how they both develop along the same path at first before diverging.

If all this were true, you might be able to test it by looking at the fossils of early relatives of remoras. Perhaps they captured the early stages of the transition.

That is precisely what Matt Friedman at the University of Oxford and his colleagues have done, as they report today in the Proceedings of the Royal Society.

Frieman is an expert on the larger group of fish to which the remora belongs, known as spiny rayed fishes. The group has evolved into some spectacularly weird forms, including sea horses and flatfishes. In 2008, I blogged about how Friedman found an intermediate flatfish, with one eye moving towards the other side. Remoras, with their own brand of weirdness, seemed to Friedman another spiny rayed fish worth spending some time on.

The only problem was that most of the fossils of remoras belonged to living lineages. Their suction disks were pretty much like what you’d find on a remora today. At least, that’s what most paleontologists who study remoras have thought.

Friedman decided to take a closer look at one of those remora fossils, called Opisthomyzon. The 30-million-year-old specimen was the first remora fossil ever found, in 1886, and it has sat in a museum in Switzerland ever since.

The specimen was in bad shape, Friedman found. It had a suction disk, for example, but it wasn’t clear if the whole disk had been pushed away from its original location. Friedman wondered if there might be other Opisthomyzon fossils hidden in other museums. Sometimes paleontologists can’t quite figure out what they’ve found, and they file away fossils without describing them.

At the National History Museum of London, Friedman found not one hidden Opisthomyzon fossil, but two. Mark Graham, a preparator at the museum, painstakingly chipped away at the underside of one of the new fossils, until all that was left was a paper-thin slab of rock. Friedman and his colleagues then compared the anatomy of Opisthomyzon to living remoras, as well as to extinct and living relatives, such as Mahi-Mahi.

Opisthomyzon proved to be exactly what Friedman was looking for: an extinct species that branched off before the origin of the living lineages of remoras. And when he and his colleagues examined its anatomy, they found exactly the kind of fish you’d expect to see from developing remoras: a fish with a suction disk still evolving from a fin.

This figure below sums up the story. The top fish is a conventional relative of remoras, with its dorsal fin bones shown to the right. In the middle is Opisthomyzon, with its corresponding suction bones. And at the bottom is a living remora.

You can see that the suction disk on Opisthomyzon is smaller than that of living remoras and does not sit over its whole head as it does today. The lamellae themselves bear more of a resemblance to the spines of dorsal fins. Opisthomyzon’s lamellae lacked a comb of spikes, for example, still retaining a single spine at the center.

Friedman’s research now gives us a richer hypothesis for how the remora got its sucker. Some of the remora’s closest living relatives, like cobia, tag along with bigger fish to scavenge on their scraps. The ancestors of remoras may have lived a similar life.

It’s not rare for spiny rayed fishes to grow extra dorsal fin spines. In the ancestors of remoras, such an anatomical fluke may have allowed them to latch their dorsal fin into the skin of a host fish, if only briefly. Even if they could spend a little time hitch-hiking this way, they would save energy that they’d otherwise have to spend on swimming for themselves.

Gradually, the remora’s dorsal fin became better adapted to latching onto other animals. As it moved towards the remora’s head, for example, it reduced drag. And as the fin bones spread outward, they attached the remora more strongly.

Sometimes, when we look at an adaptation in living animals, it seems to exquisitely well-suited to the animal’s life that we can’t imagine how a more primitive version of it could have provided any benefit. What good is half a wing, for example? What good is half a sucker? Fossils can give our limited powers of imagination a boost, by showing us that these intermediate forms did indeed exist. Opisthomyzon probably could ride on other animals, although it may have been more prone to get peeled off along the way. Remoras are so good at clamping onto their hosts that they’ve lost some of the traits that other fishes have. Their tails are weak, and they need a strong current of water passing over them in order to breathe through their gills. It’s probably no coincidence that Opisthomyzon had a much stronger tail than living remoras. It was only part-way down the road to ridiculousness.

For 10,000 years, we’ve created a new evolutionary arena where a new kind of plant has evolved: the weed. In today’s New York Times, I talk to evolutionary biologists who are studying how weeds first arose, and the marvelously devious strategies they’ve evolved to thrive on farms. You may have be seeing headlines these days about how GMO crops are creating “superweeds.” The new generation of resistant weeds is definitely a serious problem, but it’s not some new Frankensteinian phenomenon. Weeds are just doing what weeds have done so well before. Understanding their evolution may help farmers fight them more effectively and safely. Check it out.

Here we see a happy, typical family of sea monkeys. Note the red bow and plump lips that indicate the female of the species, and the tall body and protective stance of the male. I assume that the father’s well-placed tail blocks some other clues to his identity. The parallels between the sea monkeys and the human family (see inset) are uncanny and surely nothing more than a coincidence.

The real life of sea monkeys (brine shrimp, or Artemia) is a pretty far cry from Ozzie and Harriet. Sea monkeys don’t live in families, for one thing. And in a lot of populations, the females have no need for males. Their eggs can develop into healthy embryos–and, eventually, adults–without the need of sperm. You can take that picture of sea monkeys and wipe Dad out.

From an evolutionary perspective, this father-free way of life has a lot going for it. Let’s say you’ve got a sexual pair of male and female shrimp in one tank, and two asexual females in the other. Let them breed for a while. Sexual species typically produce a roughly even ratio of sons and daughters. So only half of the sexual population can produce eggs, while every individual in the asexual one can. It won’t be long before the asexual population is far bigger than the sexual one. Out in the wild, this proliferation should mean that the genes for male-free reproduction should quickly dominate populations. Down with sex, in other words.

But this has only occurred in only about one in every ten thousand species of animals. Sex must have a powerful advantage that overcomes its disadvantage–what the late biologist John Maynard Smith dubbed the two-fold cost of sex.

Scientists have given this question a lot of thought, and they’ve come up with some possible answers that they’ve been testing in recent years. Maybe sex lets adaptations evolve faster, because mothers and fathers can combine genes into new combinations. Defenses against ever-evolving parasites might be especially important. There may be different explanations for different cases. Very often, when an asexual lineage emerges, it gains an extra set of chromosomes. That’s a lot of extra DNA to build when a cell divides–which requires a lot of phosphorus and other ingredients. Perhaps that’s a cost too great to balance the advantage of giving up fathers.

Or perhaps the rarity of asexual animals is the result of evolution playing out not in short-term competitions, but over vast stretches of time. Populations of sexual animals may be less prone to going extinct because they can adapt to more niches.

To better understand the evolution of sex, a number of biologists are looking to the exceptions to the rule. If the advantages of sex overwhelm its costs for 9,999 species out of every 10,000, then why is the opposite true in the remaining one? One lineage of microscopic animals called bdelloid rotifers has been asexual for 80 million years. Cornell scientists have suggested that they have remained asexual because they’ve found a way to resist parasites that’s as good as sex–by drying up and blowing away from their pathogens.

But there are other puzzles to the evolution of sex. And one involves sea monkeys. In a paper appearing in the Journal of Evolutionary Biology, Marta Maccari of the University of Hull and her colleagues describe a massive survey of brine shrimp from across Europe and Asia. They reared cysts from dozens of populations and closely examined the offspring over the course of two generations. The females in these populations can reproduce on their own. And yet in most of the populations they studied, they discovered a few males.

The males were exceedingly rare–around one in thousand in many cases, and around one in a hundred in a few. And yet they were healthy and fertile. The males couldn’t mate with females of their own population, but they readily had sex with other species. What’s more, their hybrid offspring were healthy and fertile, too.

If asexual animal species are rare, species with asexual females and rare males are even more rare. Only a few other examples have turned up, such as certain populations of snails in New Zealand. Maccari and her colleagues don’t think there’s a clear answer to why these rare males exist. But there are a few plausible possibilities.

Maybe it’s just a fluke. It’s possible, for example, that as eggs develop, a few accidentally lose a chromosome, altering their sex. Sons, in other words, are birth defects.

It’s also possible that some of the asexual brine shrimp have mutations that lead sometimes to males, and they pass their mutated genes down to their offspring. In her study on New Zealand snails last year, Maurine Neiman of the University of Iowa and her colleagues found, surprisingly, that producing a few sons that can’t mate with any females of your species doesn’t put asexual animals at a major disadvantage.

On the other hand, maybe rare males are a side-effect of brine shrimp biology. One way for females to reproduce is to combine two eggs, joining together their chromosomes into a full set. This process can produce lots of different combinations of the shrimp’s DNA, and that variation may help them adapt to the changing environment. Sometimes, though, those combinations may produce a fertile son.

The most interesting possibility Maccari and her colleagues raise is that the rare males are a way for the genes for asexuality to spread themselves. The males can’t mate with their own species, but they can interbreed with others. They may then introduce genes for asexual reproduction into the species, causing them to turn male-free. For brine shrimp, in other words, fathers may be a way of getting rid of fathers. I have no idea how you’d paint that on a box of sea monkeys, but I’d be curious to see someone try.

For this week’s “Matter” column, I write about the bees buzzing from flower to flower this summer. In particular, I take a look at the bees that pollinate 20,000 species of plants–including crops like tomatoes and potatoes–with some amazing acoustics. They vibrate hundreds of times a second to shake pollen loose from special tubes in the flowers.

That’s why you can use a tuning fork to coax some flowers to release a cloud of pollen, as this video from Anne Leonard of the University of Nevada, Reno, illustrates. Bees, in other words, are living tuning forks.

As I continue to catch up from a week’s vacation, I realize that I neglected to point Loom readers to last week’s “Matter” column in the New York Times. It’s a fun one: a look at the species with the fewest known genes in its genome–just 120. Which raises the questions, just how low can you go? Is there some minimal essence of life? The answer is not what you might think. And it involves living inside a mealybug.

Andrew Howley over at National Geographic News Watch shares my fascination with such “Whoa…” questions, and so we exchanged some further thoughts about what it means to be alive. You can read his conversation with me here.