If you trace back the origin of plants, you end up at three separate points. That's because plant traits evolved three times, independently of each other.

	Red Hot? Red plants are closely related to unicellular red algae like Cyanidinium caldarium that blooms in the hot springs of Yellowstone National Park. Similarly, terrestrial flowering plants evolved from unicellular green algae. © Walt and Mimi Miller

At first glance, leafy kelp, sea lettuce and red algae all appear to belong to the same plant group on the evolutionary tree. But research has determined that these types of algae belong to three different groups: brown, red and green. Yet, these groups all evolved with similar characteristics.

The origin of modern plants is rooted in each of these three groups of algae. All inherited the mechanism to convert light into energy from their separate ancestors, though each group specializes in the wavelengths of light that penetrate water to the depths where it lives. Green algae, for example, grow on the water's surface to stay in direct sunlight. On the other end of the spectrum, red algae thrive on light that can beam through the water to depths of several meters. When unrelated groups with similar characteristics each emerge independently, the relationship is called polyphyletic—meaning they do not share a common origin.

	The naked truth Calcium-based sponges resemble your bath time loofah, but that's where the similarities end. First, sponges are animals, not plants. Most live sponges are delicate, and pieces that break off can develop into new sponges. Finally, natural sponges are not square; they are symmetrical around a central point. © Karen & Ian Stewart / Bruce Coleman, Inc.

EVOLUTION: The Animal Within
Perhaps the most remarkable evolutionary tidbit to be squeezed from sponges is key to our understanding of the origin of animals. Sponges don't look like most animals. They have no nervous system and no internal organs. Two separate groups of sponges exist: glassy sponges and calcium sponges. The latter is more closely related to other animals.

Sponges filter food through cells called choanocytes with waving, whiplike flagella. It turns out that these filtering cells resemble single-celled organisms called choanoflagellates, the closest relatives of animals. Comparing genetic material from sponges, other animals and choanoflagellates, revealed that sponges are a transitional kind of animal between single-celled life and animals that evolved later. This information, combined with the fact that sponges evolved into two separate groups on the evolutionary tree, means that the ancestor of all sponges is also the ancestor of all animals.

Jumbled Jellies
Jellyfish and comb jellies may look like similar creatures that float freely in the sea, but it turns out that these mysterious animals are not as closely related as you might think.

	This way up Free-floating comb jellies were the first animals to develop a gravity-sensing organ to help them stay upright. The inner ear in humans functions in a similar way. © David Wrobel / Visuals Unlimited, Inc.

Stinging cells are the most obvious characteristic that separates jellyfish and their relatives from comb jellies. Jellyfish, hydras, corals and anemones are all members of a group called Cnidaria. They have spring-loaded stinger cells called nematocysts for defense and hunting. They also live at least part of their lives as polyps that are sessile, or attached to a surface. Some, such as corals, grow as many individuals in a colony.

Comb jellies resemble jellyfish, but on closer examination they have different characteristics. They belong to a group called Ctenophora. Instead of stinging their prey, they use sticky cells to nab food. Most species are free-floating and maneuver by paddling with eight rows, or combs, of tiny cilia. Lastly, their bodies are more two-sided than symmetrical around a central point, as shown by their paired tentacles.