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

10 years

102 researchers from 21 countries

129,000 specimens

25,000 species in a 6,000-hectare forest

…yielding an estimate of 6 million arthropod species on our planet.

Ready for the details behind the numbers?

In 2003 and 2004, a large team of scientists (see numbers above) led by the Smithsonian Tropical Research Institute on an endeavor called Project IBISCA-Panama, scoured Panama’s San Lorenzo rainforest for arthropods (which includes insects, spiders, and millipedes).

They sampled the forest from top to bottom from a construction crane, inflatable platforms, and balloons, climbing ropes through forest layers as well as crawling along the forest floor to sift soil and trap arthropods.

They then spent the next eight years identifying the 129,000 specimens collected within twelve 20-by-20 meter squares. They determined that within those specimens, there were over 6,000 species of arthropods. Using various models the team extrapolated the total number of arthropod species to 25,000 residing in the 6,000-hectare forest.

The research is published in the current edition of Science.

According to Science Now,

The study is the most extensive survey of insects, spiders, and their relatives ever undertaken and should help researchers get a better understanding of what factors influence biodiversity.

“This is a high number as it implies that for every species of vascular plant, bird or mammal in this forest, you will find 20, 83, and 312 species of arthropods, respectively,” explains lead author Yves Basset.

“If we are interested in conserving the diversity of life on Earth, we should start thinking about how best to conserve arthropods,” adds Tomas Roslin, one of 35 co-authors.

“Another exciting finding was that the diversity of both herbivorous and non-herbivorous arthropods could be accurately predicted from the diversity of plants,” says Basset.

“By focusing conservation efforts on floristically diverse sites, we may save a large fraction of arthropods under the same umbrella. Further, this strengthens past ideas that we should really be basing estimates of global species richness on the number of plant species,” stresses Roslin.

For some amazing images of these arthropods and the collection process, please visit National Geographic.

Image: Thomas Martin, Jean-Philippe Sobczak, and Hendrik Dietz, T.U. Munich