How do You date the Origin of Plant life on Earth when they leave very little behind to place your calculations on?

Land plants arose earlier than thought-and may have had a bigger impact on the evolution of animals

We have land plants to thank for the oxygen we breathe. And now we have a better idea of when they took to land in the first place. While the oldest known fossils of land plants are 420 million years old, researchers have now determined that pond scum first made landfall almost 100 million years earlier.

From article, (For decades biologists have been trying to come up with a reliable birth date for land plants. Lacking backbones and hard shells, plants leave relatively little behind in the fossil record, so researchers suspect even the oldest plant fossils don't represent the first flora. 

Some scientists have tried to use plant genetic data as "molecular clocks"—knowing a typical mutation rate, they can estimate how long ago various species split based upon their differences in DNA-to figure out their evolutionary history. But they have been unable to sort out the lowest, or earliest, branches of the plant family tree. At that base, vascular plants--which include the trees, crops, and flowers we are most familiar with--came along sometime after liverworts, hornworts, and mosses. Yet the order in which those three other groups appeared has been a mystery and has stymied molecular clock studies. 

 Philip Donoghue thought that if he brought enough computer power to bear on this problem, he could solve this mystery. Donoghue, a paleobiologist at the University of Bristol in the United Kingdom, and his colleagues started with previously collected genetic data on more than 100 plant and algal species. They tested every permutation of the relationships of the four groups of plants with several kinds of these analyses and factored in the ages of dozens of plant fossils as a reality check. 

The exact configuration of the base of the plant family tree doesn't matter to dating the first land plats, Donoghue and his colleagues report today in Proceedings of the National Academy of Sciences. All the analyses indicate that land plants first appeared about 500 million years ago, during the Cambrian period, when the development of multi-cellular animal species took off. 

The new analysis “shows that the first land plants arose earlier than we thought, regardless of current uncertainties about which land plants evolved first," says Lenton.

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