The oldest land plants, mosses and lichens, appeared 470 million years ago in a desolate landscape. All land plants evolved from green algae, and the earliest plants lacked vascular tissue. Fungi were early allies of land plants, forming symbiotic relationships. The oldest known land plants with vascular tissue date back 425 million years, and the oldest land animals appeared around the same time. True forests did not appear until several tens of millions of years later.
The earliest evidence for the oldest land plants appears as small tetrad spores dated to 470 million years ago, from the Middle Ordovician Period. These came from mosses and lichens. At the time, the land would have been mostly desolate, covered in deserts and badlands, with little vegetation to be seen except for weeds at the edges of streams and ponds. Green algae took their first steps on earth, and it is from green algae that all land plants are thought to have evolved. This idea is supported by genetic and morphological studies.
In the beginning, all land plants were bryophytes (non-vascular), meaning they lacked specialized tissues to transport water and nutrients, found in most modern plants. These plants had to suck their nutrients directly from the environment or die trying. One of the early allies of the algae pioneers, the ancestors of the oldest land plants, were fungi, whose hyphae (fungal hairs) are found intermingled with these fossils. This is a lichen – a close symbiotic relationship between a fungus and a green algae. The fungus-plant symbiosis continues today, as numerous plants have fungi living in their roots, helping them draw nutrients from the soil.
The oldest known land plants with vascular tissue date from the Silurian period, about 425 million years ago, when small organisms like Cooksonia appear in the fossil record. Cooksonia was a vascular plant with simple stems displaying several Y-shaped branches as they ascend to a sporangium, a spherical packet of spores that surmounts them. The simple structure of the plant included no visible leaves or roots, although the roots may have been left out during the fossilization process. For the most part, the plant looks more like a mushroom than the green plants we’re familiar with. Regardless, it was among the oldest land plants.
At about the same time that the oldest land plants appeared, the oldest land animals began to inhabit these fledgling mini-forests. These animals included arthropods similar to present-day bristletails, early chelicerates related to modern spiders, centipedes, centipedes, and small scorpions. It would not be until the mid-Devonian, several tens of millions of years later, that the first true forests appeared on land.
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