This graph presents an estimate of Phanerozoic CO₂ levels on Earth.

From the onset of the graph, it appears that CO₂ levels are rising. However, I've come across papers, such as this one, suggesting that Proterozoic CO₂ levels were higher than those in the Phanerozoic.

I'm seeking a summary of how multicellular life has influenced Earth's CO₂ levels to better understand our planet. I recognize that this topic is vast and could span an entire book. So, to narrow it down:

I understand that both foraminifera and other unicellular and multicellular organisms have been storing CO₂ since the beginning of the Phanerozoic.

On the other hand, multicellular organisms, especially plants, have acted as CO₂ sinks. This began during the Devonian-Carboniferous period. However, data suggests a declining CO₂ trend since the Cambrian. This leads me to believe that the decrease in atmospheric CO₂ throughout half of the Phanerozoic might be mainly attributed to shell-forming organisms. This brings me to a central question:

Which removes more CO₂, plants or shell organisms?

I'm curious about how post-Cambrian life impacts the global Earth's carbon cycle. Is it primarily consuming carbon? This makes me wonder if, on this and potentially other planets, without the intervention of multicellular organisms with advanced cognition, the survival and progression of C-based life would be tied to the availability of carbon in the atmosphere. Would this then be another gift we received that is not for every times the planet star is alive?

Additionally, I'd appreciate it if you could answer this other question I've posted with a bounty.

_{*Text corrected using GPT4}