As early vertebrates emerged from the water, their tails may have played a crucial role in helping them move across land, a new study reports. The results, based on animal and robot models, as well as mathematical analysis, may shed light on the origins of terrestrial life on Earth. Life first dwelled in the watery depths of the oceans - until roughly 385 to 360 million years ago when early tetrapods made a move to land. These animals, which were adapted for water, would have had to find unique ways to move their bodies across land - and the nature of shorelines, with granular surfaces and sloping banks, only added to the challenge. To gain more insights into the early locomotion required for this transition, Benjamin McInroe and colleagues studied the movement of the modern mudskipper, a fish that sometimes enjoys a "stroll" on land using its fins, and occasionally jumps using its tail. The researchers found that on flat land, using its tail only provides minimal benefit for lateral movement. Yet, as the slope of a surface increased - as one might see along a riverbank - the tail became significantly more important in helping the mudskipper propel itself forward. For example, at slopes of 10° the tail was used in roughly one third of all "steps," and at slopes of 20° it was used during more than half of all steps. On flat ground, the tail was rarely used. The team created a robot that mimics various movements of the mudskipper and early tetrapods, manipulating its limbs to better understand how the fins and tails move in concert to propel a fish forward on land. On flat land, the angle at which the robot could move its "fins" towards its body was key; at higher slopes, similar to the mudskipper, the tail became a dominant factor for lateral movement, the authors show. The tail was also useful for anchorage, so that the robot didn't slide backwards down the slope. A Perspective by John Nyakatura discusses the study in more detail.