In 1974, a group of scientists broadcast a message towards Globular Cluster Messier 13 with the Arecibo telescope. The message was crafted in such a way that any aliens receiving it would potentially be able to understand some basic information about humanity.
The components
The wikipedia article is great and goes into detail about how the components are meant to be interpreted, but I'll briefly summarize their purpose here (from top to bottom, with colors matching the image):
1. The numbers one through ten
2. Chemical elements in DNA
3. DNA backbone with two base pairs
4. The estimated number of base pairs in the human genome
5. A picture of a double helix
6. (left) A ruler to indicate the human's height
7. (left) The number 14 (when multiplied by the wavelength of the message, it gives the height of the average human male)
8. (center) A picture of a human
9. (right) The estimated population of humans in 1974
10. The major bodies of the solar system, with Earth raised
11. A picture of the telescope
12. Unclear - maybe this indicates the ground?
13. The number 2,430 (when multiplied by the wavelength, it gives the diameter of the telescope)
The problem
The technology of the time made it impossible to accurately measure the number of base pairs in the human genome, and the value they used turns out to have overestimated it by over 37%. It was also coincidentally extremely close to the then-estimated population, which I can only imagine would send our alien interlocutors down a rabbit hole. The full human reference genome sequence was (perhaps surprisingly) only finally determined in August 2023 (see this nice overview that covers how the reference genome has improved over the last few decades).
Updating the Arecibo message
I wrote a CLI tool to generate the message with user-provided values for the genome size as well as the population, which has obviously changed. On the left, I've highlighted the components being updated (blue is the genome size, red is the population), and on the right, we see the message if it were being sent today, with 3,117,275,501 base pairs and 8,098,171,861 humans (the estimated population by the US Census at the time of this writing):
Buffer overflows and Pluto
The human population is represented by a binary number read from left to right, top to bottom, with the least-significant digit coming first. The largest value this could ever hold (see image on the left) without expanding into the depiction of the solar system would be 281,474,976,710,655 (unless we destroy every planet after Jupiter, which would give us an additional five rows of six bits, for a total of 302,231,454,903,657,293,676,543) - either way, we'll be able to continue using this format to alert aliens to our presence for many years to come. To accommodate the transition that occurred in 2006, the CLI tool also has a --pluto-is-not-a-planet flag that disables Pluto (right).
The CLI
Install: cargo install modern-arecibo
Repo: https://github.com/jimrybarski/modern-arecibo
Crate: https://crates.io/crates/modern-arecibo