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Plant Evolution: Where do trees come from?

Second only to their distant cousins, the bacteria, plants are among the most successful type of life found on this planet. They colonized the sub-arctic millions of years before animals (including us) ever evolved and have survived several mass extinctions that killed off some of the most successful organisms to have ever inhabited this planet.

Prokaryotes: Where is all began

Made up of a small, microscopic cell, prokaryotes are the oldest and most successful organism to have ever inhabited our Earth. They have been found in fossils dating back 3.5 billion years and make up 90% of the biomass found in our oceans, yet, they are too small to see with the naked eye. We owe them an enormous debt of gratitude as, without their ability to convert Carbon Dioxide into Oxygen, life never would have emerged from the ocean of our planet. Prokaryotes can be found on ever part of our planet form the deepest oceans to the highest peaks across every environmental extreme, including within our own body!

Prokaryotes evolving into Eukaryotes (plants)

Where prokaryotes are simply living cells, eukaryotes have the added ability to get some if not all of their energy from the Sun through a process known as photosynthesis. Eukaryotes, evolved from prokaryotes between 1 and 2.5 billion years ago through a process known as endosymbiosis in which a photosynthetic cell (chloroplasts) was able to be incorporated within a bacteria. This was a breakthrough as now the cell would begin to gain the ability to power itself simply using the energy given off by the Sun. Keep in mind, we are still dealing with small, mostly invisible cells… we are nowhere near the “plant” stage yet.

Eukaryotes evolving into Algae

For well over a billion years, photosynthetic cells (eukaryotes) stayed at sea due to the intensity of UV rays that existed, anything that left the ocean would instantly fry. You see, back then, there wasn’t an Ozone layer protecting us from the Sun’s rays nor any stores that sold sunscreen for these microscopic cells, this was built, one oxygen molecule at a time, by the hard work of eukaryotes over billions of years. As evolution continued, these eukaryotes evolved into algae that was able to live in fresh water, survive drought as they slowly populated every surface that was within spitting distance of a water source. Rain, wind and geological processes where the only dispersal method used by these algae, which are still with us even today!

Where did plants come from?

Just reflect for a moment, well before any living thing existed on the planets surface, there was ocean and bare rock, that’s it! No soil, no leaves, no twigs, not even bugs! These powerful little algae where Earths first pioneers, they slowly emerged from the worlds oceans, conquering first fresh water, then dispersing slowly across every surface around the world. They had the entire planet to themselves! If you could travel back in time, all you would see over most of the Earths surface not covered by ocean is a vast carpet of green microscopic life when it rained! These algae, my friends, are the first hint of plants to come.

Over several hundreds of millions of years, these algae slowly evolved and diverged into cyanobacteria, lichens then seedless bryophytes (liverwarts and mosses) which lack anything resembling what most people would classify as a plant. Fast forward even more to about 425 million years ago when the first vascular plants began to develop. These plants where only a few inches high, suffered in an incredibly difficult environment and could only grow where water was present. You see, bryophytes do not have the ability to store water so they are 100% dependent on the environment to keep them alive if they left the safety of a water source to go more inland. As a result, they are defined as poikilohydric organisms.

Quick Review

Let’s do a quick review, based on the fossil record, a prokaryote met up with a chloroplast billions of years ago and enter in a mutually beneficial relationship as a eukaryote. This eukaryote led to the birth of algae which then, after billions of years producing oxygen, CHANGED our planet to the point where anything leaving the safety of water wouldn’t get zapped by UV rays. Yes, a microscopic organism modified an ENTIRE PLANET so that it could support life on it’s surface. AMAZING! The oceans where teeming with life, that environment was a battlefield of evolution, new species evolved while others became extinct, the competition was fierce! Now, we got algae who discover that life on the planets surface is far easier and go gangbusters as a result creating the next stage, bryophytes!

Plants evolve from poikilohydric to homoiohydric organisms

Up to this point, when ever it rained on the Earth’s surface, everything came back to life, after which, everything quickly dried out and went back into dormancy. It was an extreme of boom / bust cycles similar to what you see on the African Serengeti right before the rainy season hits. Trees, if you ever noticed, don’t dry out, they can store water and control their moisture loss, as such, they are referred to as homoiohydric plants. The ability for plants to store their own water supply is thought to have evolved around 400 million years ago, plus / minus a few tens of millions of years.

Where did trees come from?

Once plants where able to store their own water supply in between rains, the only limiting factor left to them was that of sun. You see, if they wanted to grow bigger, they had to grow faster then their closest neighbor before they blocked out the sun from them. As a result, plants that where able to store more water and grow at an ever increasing rate of efficiency gained a strategic advantage over their other neighbors. The end result was an arms race to see who could grow the fastest to dominate the treetop canopy. These early trees, for the most part, didn’t reproduce by using seed dispersal but by spores or other clonal means. These guys where rather small because they couldn’t produce woody tissues. They would quickly grow and fall due to their own weight or even with the slightest of winds as they lacked support tissues that give modern day trees their incredible strength. As such, they where referred to as progymnosperms. Now, these trees are not trees in the traditional sense, they are more closely related to your neighborhood ferns then the mighty Oak. We still don’t have cedars, maples nor oaks in these forest, they don’t evolve for a few million years to come.

Where did ancient trees come from?

Well, ancient trees, called gymnosperms due to their ability to produce seeds for reproduction, don’t show up in the fossil record until 365 million years ago, again, plus or minus a few tens of million of years. They evolved, you guessed it, from their very week cousins, the progymnosperms. These guys where incredibly hardy, they could store vast amounts of water, grow taller then any progymnosperm could ever imagine and due to their ability to pollinate, able to greatly speed up their diversification across the rest of the earth’s surface. Having seeds that could be carried by water, wind and later, other animals or insects also allowed them to achieve incredible reproductive success.

Plant reproduction: Seeds vs Spores

Your next question may be, why the heck did the trees that had the ability to produce seeds (gymnosperms) quickly beat out those that couldn’t (progymnosperms)? Well, for one, if you are reproducing using spores, you can only develop in a wet environment where timing is critical as there aren’t any reserves so photosynthesis is a must right away! Seed producing plants, on the other hand, gave their young plants a bucket of nutrients to meet their growing needs, this not only helped them gain a foothold in harsher environments but also allowed them an immediate boost at the beginning of their life cycle.

Where did modern trees come from?

Wow… I bet you learned a lot of background about the wood laying around in your wood shop, now for the more relevant part. Where the heck did that Birch, Redwood, Pine, Cherry, Ash, Spruce, Oak, Poplar, Walnut, Elm, Beech or even Douglas Fir laying in your wood bin come from? There are more tree types out there then letters in the alphabet! Well, many of these guys are gymnosperms (seed producing) and have been on the scene for several hundreds of millions of years but for others, the ones that require pollination, they are just starting to show up around 150 million years ago. Before this time, plants produced seeds that where cross-pollinated on an continental scale, if you had a pollen allergy, our planet would have been a living hell! These plants are referred to as angiosperms and this is a chicken or the egg riddle in which you have to ask, which came first… the pollinators or the flowers? That is a question for another time but suffice it to say. This is when trees that bear fruit (Cherry, Apple and Pear trees) came onto the scene. Instead of a shotgun approach towards reproduction, angiosperms use other organisms and the wind to help with fertilization. This saves them a kings ransom in resources and also greatly increased their reproductive success.

Summary

Here is a rundown of how things happened to produce that fine piece of Cherry you are about to build a wood marvel with.

Prokaryotes (bacteria) –> Eukaryotes (plants) –> Algae –> bryophytes (few inches high) –> poikilohydric (can’t store water) –> homoiohydric (can store water) –> progymnosperms (spore) –> gymnosperms (seed) –> angiosperms (flowering)

The Bottom Line

Plants pre-date humans by hundreds of millions of years, give it the respect it has earned.

Plant Evolution