The period on our planet between 540 and 520 million years ago when most modern animal groups appeared is also known as evolution’s Big Bang. Prior to the Cambrian explosion, life was much simpler on Earth—single-celled organisms dominated the landscape.

But how did so many different organisms develop in such a short period of time? “The abrupt appearance of dozens of animal groups during this time is arguably the most important evolutionary event after the origin of life,” says Michael Lee of the University of Adelaide. “Darwin himself famously considered that this was at odds with the normal evolutionary processes.”

Lee and his colleagues decided to look into “Darwin’s dilemma,” focusing on arthropods (insects, crustaceans, arachnids and their relatives), the most diverse animal group in both the Cambrian period and present day.

“It was during this Cambrian period that many of the most familiar traits associated with this group of animals evolved, like a hard exoskeleton, jointed legs, and compound (multi-faceted) eyes that are shared by all arthropods,” explains team member Greg Edgecombe of the Natural History Museum of London. “We even find the first appearance in the fossil record of the antenna that insects, millipedes and lobsters all have, and the earliest biting jaws.”

The team quantified the anatomical and genetic differences between living animals, and established a timeframe over which those differences accumulated with the help of the fossil record and intricate mathematical models.

“In this study we’ve estimated that rates of both morphological and genetic evolution during the Cambrian explosion were five times faster than today—quite rapid, but perfectly consistent with Darwin’s theory of evolution,” Lee says.

ScienceNOW offers the numbers:

The creatures’ genetic codes were changing by about .117% every million years—approximately 5.5 times faster than modern estimates.

Unusual, perhaps, but in line with natural selection, the team indicates. The study appears in the recent edition of Current Biology.

Perhaps Darwin can get some rest now.

Image: Michael Lee

With headlines like, “Poorly-endowed barnacles overthrow 150-year-old belief,” this recent bit of research tugged at our heart-strings. So we consulted with our barnacle expert, Academy Research Fellow Bob Van Syoc, to understand the reproductive challenges facing this arthropod.

First thing you have to know about barnacles—they’re functionally sequential hermaphrodites. “That is, they are hermaphrodites that act as either male or female at any point in time, not both at the same time,” Van Syoc explains. “In all of these, ‘pseudo-copulation’ occurs by the transfer of sperm from the penis of the male inserted into the female or hermaphrodite mantle chamber where the eggs are fertilized and brooded until larvae hatch.”

Sounds like fun, right? And unlike the poor barnacle in the National Geographic blog title above, most barnacles are actually quite well-endowed. From ScienceNOW:

Even Charles Darwin marveled at the length of the barnacle’s penis. In some species, it’s up to eight times the body length.

Sadly, that’s not the case for the Pacific gooseneck barnacle, Pollicipes polymerus, the poorly-endowed barnacle of the title. And that’s not all. “As a stalked barnacle with a flexible peduncle it is relatively less evolved than the sessile, volcano-shaped shelled barnacles that are most common on our shores,” says Van Syoc.  “P. polymerus is a member of a relict genus that exists only in the eastern boundaries of the Pacific and Atlantic Oceans.”

Perhaps that’s why, size and all, these barnacles “spermcast,” according to a surprising study last month in the Proceedings of the Royal Society B. Spermcasting is just as it sounds—releasing sperm into the water column hoping for a bit of luck. For most barnacles, this doesn’t produce the best results.

Here’s how Van Syoc describes it: “Broadcast fertilization (or ‘spermcasting,’ as the authors have it) is wasteful of gametes and would not be expected in an efficient evolutionary sequence once internal fertilization has evolved.  However, it does increase the chance of sperm reaching a target egg and greatly increases the reach of individuals and their ability to fertilize more than a few individuals nearby.”

That P. polymerus spermcasts would have shocked even Darwin, who wrote quite a bit about barnacles and barnacle sex.

“The study challenges our traditional (i.e., since Darwin!) ideas of barnacle mating and fertilization,” Van Syoc says. But he likes the study and techniques the paper’s authors used. “It illustrates that field ecology observations (solitary individuals brooding embryos) coupled with modern DNA analysis techniques to help determine parentage can, and should, be used in concert. Too often, scientists either eschew new techniques as ‘trendy’ or traditional methods as ‘old fashioned.’ Here we have a great example of how they can work together to great effect.”

Image: Minette Layne/Wikipedia